The different sects or denominations of Judaism are generally referred to as movements. The differences between Jewish movements today are not so much a matter of theology, but more a matter of how literally they take the scriptures, how much they think biblical requirements can be changed, and whether those requirements are mandatory. I've been told that the differences between Jewish movements are not as great as the differences between Christian denominations, but I'm not sure if that's true: I once heard a Protestant minister trying to explain to Jews the difference between Protestant denominations, and the first distinction he thought of was the country of origin of the adherents, which doesn't seem very significant to me but maybe he was missing the point of the question.
In general, when I speak of "movements" on this site, I am referring to movements in the United States in the 20th and 21st centuries, but in fact there have been organized differences of opinion for more than 2000 years.
Perhaps the oldest records we have of a formal difference of religious opinion among Jews dates back to the time of the Maccabean revolt, which is the basis for the story of Chanukkah. At that time, the land of Israel was under the relatively benevolent control of Greece, and was deeply influenced by Greek culture. Hellenizing Jews were opposed by a religious traditionalist group known as the Chasideans (no direct relation to the modern movement known as Chasidism). As the Seleucid Greeks began to oppress the Jews, war broke out and the Jewish people united in their opposition to the Greeks.
The war continued for 25 years, and the Jewish people remained united in purpose. But after the war ended, the Jewish people became divided into three groups: the Essenes, the Sadducees (Tzedukim in Hebrew) and the Pharisees.
The Essenes were an ascetic and mystical group devoted to strict discipline. They lived in isolation from the world. The Dead Sea Scrolls are believed to be the product of an Essene sect. Some scholars believe that early Christianity was influenced by the mystical and hermetical teachings of the Essenes.
The Sadducees evolved out of the Hellenistic elements of Judaism. The movement was made up of the priests and the aristocrats of Jewish society. They were religiously conservative but socially liberal. The Sadducees believed in a strict, narrow and unchanging interpretation of the written Torah, and they did not believe in oral Torah. The Temple and its sacrificial services were at the center of their worship. Socially, they adopted the ways of the neighboring Greek culture.
The Pharisees believed that G-d gave the Jews both a written Torah and an oral Torah, both of which were equally binding and both of which were open to interpretation by the rabbis, people with sufficient education to make such decisions. The Pharisees were devoted to study of the Torah and education for all.
After Judea was conquered by Rome and tensions with Rome began to mount, a fourth group appeared: the Zealots. The Zealots were basically a nationalistic movement, not a religious one. They favored war against Rome, and believed that death was preferable to being under Roman control. They would commit suicide rather than be taken prisoner. The most famous example of the Zealots was the defenders of Masada, who held the mountain fortress against the Roman Tenth Legion for months and ultimately committed suicide rather than surrender.
The Pharisaic school of thought is the only one that survived the destruction of the Temple. The Zealots were killed off during the war with Rome. The Sadducees could not survive without the Temple, which was the center of their religion. The Essenes, who were never very numerous, were apparently killed off by the Romans (they were easily recognizable in their isolated communities). Some think that the Essenes may have been absorbed into Christianity, which as I said shares some of their mystical teachings.
For many centuries after the destruction of the Temple, there was no large-scale, organized difference of opinion within Judaism. Judaism was Judaism, and it was basically the same as what we now know as Orthodox Judaism. There were some differences in practices and customs between the Ashkenazic Jews of Central and Eastern Europe and the Sephardic Jews of Spain and the Middle East, but these differences were not significant. See Ashkenazic and Sephardic Jews.
During the 9th century C.E., a number of sects arose that denied the existence of oral Torah. These sects came to be known as Karaites (literally, People of the Scripture), and they were distinguished from the Rabbanites or Rabbinical Judaism.
The Karaites believed in strict interpretation of the literal text of the scripture, without rabbinical interpretation. They believed that rabbinical law was not part of an oral tradition that had been handed down from G-d, nor was it inspired by G-d, but was an original work of the sages. As such, rabbinical teachings are subject to the flaws of any document written by mere mortals.
The difference between Rabbanites and Karaites that is most commonly noted is in regard to Shabbat: the Karaites noted that the Bible specifically prohibits lighting a flame on Shabbat, so they kept their houses dark on Shabbat. The Rabbanites, on the other hand, relied upon rabbinical interpretation that allowed us to leave burning a flame that was ignited before Shabbat. Karaites also prohibited sexual intercourse on Shabbat, while Rabbanites considered Shabbat to be the best time for sexual intercourse. The Karaites also follow a slightly different calendar than the Rabbanites.
According to the Karaites, this movement at one time attracted as much as 40% of the Jewish people. Today, Karaites are a very small minority, and most Rabbinical Jews do not even know that they exist. For more information about the Karaites, see The Karaite Korner.
In the 1700s, the first of the modern movements developed in Eastern Europe. This movement, known as Chasidism, was founded by Israel ben Eliezer, more commonly known as the Baal Shem Tov (Master of the Good Name) or the Besht (an acronym of Baal SHem Tov). Before Chasidism, Judaism emphasized education as the way to get closer to G-d. Chasidism emphasized other, more personal experiences and mysticism as alternative routes to G-d. This was a very popular idea because, contrary to common belief, not all Jews are intellectuals!
Chasidism was considered a radical movement at the time it was founded. There was strong opposition from those who held to the pre-existing view of Judaism. Those who opposed Chasidism became known as mitnagdim (opponents), and disputes between the Chasidim and the mitnagdim were often brutal. Today, the Chasidim and the mitnagdim are relatively unified in their opposition to the liberal modern movements. Orthodoxy and even the liberal movements of Judaism today have been strongly influenced by Chasidic teachings.
Chasidic sects are organized around a spiritual leader called a Rebbe or a tzaddik, a person who is considered to be more enlightened than other Jews. A Chasid consults his Rebbe about all major life decisions.
Chasidism continues to be a vital movement throughout the world. The Lubavitcher Chasidim are very vocal with a high media presence (see their website, Chabad.org), but there are many other active Chasidic sects today. For example, Breslov, Satmar and Bobover.
Approximately 5.7 million of the world's 14.8 million Jews live in the United States. The only country with a higher Jewish population is Israel (below) at 6.3 million. There are basically three major movements in the U.S. today: Reform, Conservative and Orthodox. Some people also include a fourth movement, the Reconstructionist movement, although that movement is substantially smaller than the other three. Orthodox and sometimes Conservative are described as "traditional" movements. Reform, Reconstructionist, and sometimes Conservative are described as "liberal" or "modern" movements. And a lot of American Jews (about 35%) identify themselves as "just Jewish" with no affiliation with any movement.
Orthodoxy is actually made up of several different groups. It includes the modern Orthodox, who have largely integrated into modern society while maintaining observance of halakhah (Jewish Law), the Chasidim, who live separately and dress distinctively (commonly, but erroneously, referred to in the media as the "ultra-Orthodox"), and the Yeshivish Orthodox, who are neither Chasidic nor modern. The Orthodox movements are all very similar in belief, and the differences are difficult for anyone who is not Orthodox to understand. They all believe that G-d gave Moses the whole Torah at Mount Sinai. The "whole Torah" includes both the Written Torah (the Jewish Bible, what Christans call the "Old Testament") and the Oral Torah, an oral tradition interpreting and explaining the Written Torah. They believe that the Torah is true, that it has come down to us intact and unchanged. They believe that the Torah contains 613 mitzvot binding upon Jews but not upon non-Jews. This web site is written primarily from the modern Orthodox point of view. The 2017 report of America's Changing Religious Identity produced by PRRI found that 10% of Americans who identify as Jewish by religion (rather than just by culture) identify as Orthodox.
Reform Judaism does not believe that the Torah was written by G-d. The movement accepts the critical theory of Biblical authorship: that the Bible was written by separate sources and redacted together. Reform Jews do not believe in observance of commandments as such, but they retain much of the values and ethics of Judaism, along with some of the practices and the culture. The original, basic tenets of American Reform Judaism were set down in the Pittsburgh Platform. Many non-observant, nominal, and/or agnostic Jews used to identify themselves as Reform when pressed to specify simply because Reform is the most liberal movement, but that is not really a fair reflection on the movement as a whole. There are plenty of Reform Jews who are religious in a Reform way. In recent years, those non-religious people have tended to identify themselves as "just Jewish," which gives a more accurate reflection of Reform Judaism. The PRRI report found that 28% of American Jews by religion identify themselves as Reform, which is down from the past but probably because the "just Jewish" have been filtered out. There are approximately 900 Reform synagogues in the United States and Canada. For more information about Reform Judaism, see The Union for Reform Judaism.
Conservative Judaism grew out of the tension between Orthodoxy and Reform. It was formally organized as the United Synagogue of Conservative Judaism in by Dr. Solomon Schechter in 1913, although its roots in the Jewish Theological Seminary of America stretch back into the 1880s. Conservative Judaism maintains that the truths found in Jewish scriptures and other Jewish writings come from G-d, but were transmitted by humans and contain a human component. Conservative Judaism generally accepts the binding nature of halakhah, but believes that the Law should change and adapt, absorbing aspects of the predominant culture while remaining true to Judaism's values. In my experience, there is a great deal of variation among Conservative synagogues. Some are indistinguishable from Reform, except that they use more Hebrew; others are practically Orthodox, except that men and women sit together. Some are very traditional in substance, but not in form; others are traditional in form but not in substance. This flexibility is deeply rooted in Conservative Judaism, and can be found within their own Statement of Principles, Emet ve-Emunah. The PRRI report found that 14% of American Jews by religion identify themselves as Conservative. There are approximately 600 Conservative synagogues in the world today.
Reconstructionist Judaism is theoretically an outgrowth of Conservative, but it doesn't fit neatly into the traditional/liberal, observant/non-observant continuum that most people use to classify movements of Judaism. Reconstructionists believe that Judaism is an "evolving religious civilization." They do not believe in a personified deity that is active in history, and they do not believe that G-d chose the Jewish people. From this, you might assume that Reconstructionism is to the left of Reform; yet Reconstructionism lays a much greater stress on Jewish observance than Reform Judaism. Reconstructionists observe the halakhah if they choose to, not because it is a binding Law from G-d, but because it is a valuable cultural remnant. Reconstructionism is a very small movement but seems to get a disproportionate amount of attention, probably because there are a disproportionate number of Reconstructionists serving as rabbis to Jewish college student organizations and Jewish Community Centers. In fact, even the Conservative movement now allows affiliated synagogues to use a Reconstructionist rabbi if they cannot find an acceptable Conservative one, and Reconstructionist rabbis at Conservative synagogues are becoming increasingly common. Everyone I know seems to have had a Reconstructionist rabbi at some time, usually in college or at a community center, yet according to the PRRI report, only about 2% of the Jews in America identify themselves as Reconstructionist. There are about a hundred Reconstructionist synagogues world-wide. See the homepage of the Jewish Reconstructionist Movement.
Though most Jews do not have any theological objections to praying in the synagogues of other movements, liberal services are not "religious" enough or "Jewish" enough for traditional Jews, and traditional services are too long, too conservative, and often basically incomprehensible to liberal Jews (because traditional services are primarily, if not exclusively, in Hebrew). Some Orthodox will not attend liberal services because of the mixed seating arrangements and because the liberal prayer book cuts or changes many required prayers.
I have been to services in Reform, Conservative, and Orthodox synagogues, and to services run by a Reconstructionist rabbi, and I have found that while there are substantial differences in length, language, and choice of reading materials, the overall structure is surprisingly similar. See Jewish Liturgy for more information about prayer services.
Approximately 5 million Jews live in Israel. Orthodoxy is the only movement that is formally and legally recognized in Israel. Until very recently, only Orthodox Jews could serve on religious councils. The Orthodox rabbinate in Israel controls matters of personal status, such as marriage, conversion and divorce.
The other American movements have some degree of presence in Israel, but for the most part, Israelis do not formally identify themselves with a movement. Most Israelis describe themselves more generally in terms of their degree of observance, rather than in terms of membership in an organized movement.
More than half of all Israelis describe themselves as hiloni (secular). About 15-20 percent describe themselves as haredi (ultra-Orthodox) or dati (Orthodox). The rest describe themselves as masorti (traditionally observant, but not as dogmatic as the Orthodox). It is important to remember, however, that the masorti and hiloni of Israel tend to be more observant than their counterparts in America. For example, the hiloni of Israel often observe some traditional practices in a limited way, such as lighting Shabbat candles, limiting their activities on Shabbat, or keeping kosher to some extent, all of which are rare among American Reform Jews, and unheard of among American Jews who describe themselves as secular. It has been said that most Israelis don't belong to a synagogue, but the synagogue they don't belong to is Orthodox.
There are an estimated 350,000 Jews in the UK. Of those, approximately 20% are Reform or Liberal, which are two separate movements. There is also a small but active Conservative movement called the Masorti, which uses the same prayer book as the Conservative movement in the United States. The Lubavitcher Chasidim are also active and growing in the UK.
The liberal movements in the UK are generally more traditional than the Reform movement in the United States. For example, the British Reform movement does not accept patrilineal descent (although the Liberal movement does). See Who Is a Jew.