Judaism is not just a set of beliefs about G-d, man and the universe. Judaism is a comprehensive way of life, filled with rules and practices that affect every aspect of life: what you do when you wake up in the morning, what you can and cannot eat, what you can and cannot wear, how to groom yourself, how to conduct business, who you can marry, how to observe the holidays and Shabbat, and perhaps most important, how to treat G-d, other people, and animals. This set of rules and practices is known as halakhah.
The word "halakhah" is usually translated as "Jewish Law," although a more literal (and more appropriate) translation might be "the path that one walks." The word is derived from the Hebrew root Hei-Lamed-Kaf, meaning to go, to walk or to travel.
Some non-Jews and non-observant Jews criticize this legalistic aspect of traditional Judaism, saying that it reduces the religion to a set of rituals devoid of spirituality. While there are certainly some Jews who observe halakhah in this way, that is not the intention of halakhah, and it is not even the correct way to observe halakhah.
On the contrary, when properly observed, halakhah increases the spirituality in a person's life, because it turns the most trivial, mundane acts, such as eating and getting dressed, into acts of religious significance. When people write to me and ask how to increase their spirituality or the influence of their religion in their lives, the only answer I can think of is: observe more halakhah. Keep kosher or light Shabbat candles, pray after meals or once or twice a day. When you do these things, you are constantly reminded of your relationship with the Divine, and it becomes an integral part of your entire existence.
Are these laws sometimes inconvenient? Yes, of course. But if someone you care about -- your parent, your child, your spouse -- asked you to do something inconvenient or unpleasant, something you didn't feel like doing, you would do it, wouldn't you? It is a very shallow and meaningless kind of love if you aren't willing to do something inconvenient for the one you love. How much more so should we be willing to perform some occasionally inconvenient tasks that were set before us by our Creator, who assigned those tasks to us for our own good?
Halakhah comes from three sources: from the Torah, from laws instituted by the rabbis and from long-standing customs. Halakhah from any of these sources can be referred to as a mitzvah (commandment; plural: mitzvot). The word "mitzvah" is also commonly used in a casual way to refer to any good deed. Because of this imprecise usage, sophisticated halakhic discussions are careful to identify mitzvot as being mitzvot d'oraita (an Aramaic word meaning "from the Torah") or mitzvot d'rabbanan (Aramaic for "from the rabbis"). A mitzvah that arises from custom is referred to as a minhag. Mitzvot from all three of these sources are binding, though there are differences in the way they are applied (see below).
Some of the mitzvot d'oraita are clear, explicit commands in the text of the Torah (thou shalt not murder; you shall write words of Torah on the doorposts of your house), others are more implicit (the mitzvah to recite grace after meals, which is inferred from "and you will eat and be satisfied and bless the L-rd your G-d"), and some can only be ascertained by deductive reasoning (that a man shall not commit incest with his daughter, which is deduced from the commandment not to commit incest with his daughter's daughter).
Some of the mitzvot overlap; for example, there is a commandment to rest on Shabbat and a separate commandment not to do work on Shabbat.
Although there is not 100% agreement on the precise list of the 613 (there are differences in the way some lists divide related or overlapping mitzvot), there is complete agreement that there are 613 mitzvot. This number is significant: it is the numeric value of the word Torah (Tav = 400 + Vav = 6 + Reish = 200 + Hei = 5), plus 2 for the two mitzvot whose existence precedes the Torah: I am the L-rd, your G-d and You shall have no other gods before Me. (Talmud Makkot 23b). The 613 are often referred to as the taryag mitzvot, because the standard way of writing the number 613 in Hebrew is Tav (400) Reish (200) Yod (10) Gimel (3). The most accepted list of the 613 mitzvot is Rambam's list in his Mishneh Torah. In the introduction to the first book of the Mishneh Torah, Rambam lists all of the mitzvot, then proceeds to divide them up into subject matter categories. See List of the 613 Mitzvot.
There is also complete agreement that these 613 mitzvot can be subdivided into 248 "positive" mitzvot and 365 "negative" mitzvot. Positive mitzvot are commandments to do something, such as the commandment to honor your mother and father. In Hebrew, these are called mitzvot aseh (commandments to do). Negative mitzvot are commandments not to do something, such as the commandment not to murder. In Hebrew, these are called mitzvot lo ta'aseh (commandments not to do). The Talmud explains that these numbers have significance: there are 365 days in the solar year, and 248 bones of the human male body (Makkot 23b). (Note: the Hebrew term translated as "bones" includes some additional body parts, which explains the discrepancy from modern medicine's count of 206 bones). Ancient sources also indicate that there are 365 sinews in the body, and a significant 248-day cycle of the moon, so both numbers have both anatomical and astronomical significance.
Many of these 613 mitzvot cannot be observed at this time for various reasons. For example, a large portion of the laws relate to sacrifices and offerings, which can only be made in the Temple, and which does not exist today. Some of the laws relate to the theocratic state of Israel, its king, its supreme court, and its system of justice, and cannot be observed because the theocratic state of Israel does not exist today. In addition, some laws do not apply to all people or places. Agricultural laws only apply within the state of Israel, and certain laws only apply to kohanim or Levites. The 19th/20th century scholar Rabbi Israel Meir Kagan, commonly known as the Chafetz Chayim, identified 77 positive mitzvot and 194 negative mitzvot which can be observed outside of Israel today.
In addition to the laws that come directly from Torah (d'oraita), halakhah includes laws that were enacted by the rabbis (d'rabbanan). These rabbinic laws are still referred to as mitzvot (commandments), even though they are not part of the original 613 mitzvot d'oraita. Mitzvot d'rabbanan are considered to be as binding as Torah laws, but there are differences in the way we apply laws that are d'oraita and laws that are d'rabbanan (see below).
Mitzvot d'rabbanan are commonly divided into three categories: gezeirah, takkanah and minhag.
A gezeirah is a law instituted by the rabbis to prevent people from accidentally violating a Torah mitzvah. We commonly speak of a gezeirah as a "fence" around the Torah. For example, the Torah commands us not to work on Shabbat, but a gezeirah commands us not to even handle an implement that you would use to perform prohibited work (such as a pencil, money, a hammer), because someone holding the implement might forget that it was Shabbat and perform prohibited work. The word is derived from the root Gimel-Zayin-Reish, meaning to cut off or to separate.
A takkanah is a rule unrelated to biblical laws that was created by the rabbis for the public welfare. For example, the practice of public Torah readings every Monday and Thursday is a takkanah instituted by Ezra. The "mitzvah" to light candles on Chanukkah, a post-biblical holiday, is also a takkanah. The word is derived from the Hebrew root Tav-Qof-Nun, meaning to fix, to remedy or to repair. It is the same root as in "tikkun olam," repairing the world, or making the world a better place, an important concept in all branches of Judaism.
Some takkanot vary from community to community or from region to region. For example, around the year 1000 C.E., a Rabbeinu Gershom Me'or Ha-Golah instituted a takkanah prohibiting polygyny (multiple wives), a practice clearly permitted by the Torah and the Talmud. This takkanah was accepted by Ashkenazic Jews, who lived in Christian countries where polygyny was not permitted, but was not accepted by Sephardic Jews, who lived in Islamic countries where men were permitted up to four wives.
Minhag is treated as a category of mitzvot d'rabbanan (from the rabbis), mostly because it is clearly not d'oraita (from the Torah), but minhag is generally not the sort of rule that is created by reasoned decision-making. A minhag is a custom that developed for worthy religious reasons and has continued long enough to become a binding religious practice. For example, the second, extra day of holidays was originally instituted as a gezeirah, so that people outside of Israel, not certain of the day of a holiday, would not accidentally violate the holiday's mitzvot. After the mathematical calendar was instituted and there was no doubt about the days, the added second day was not necessary. The rabbis considered ending the practice at that time, but decided to continue it as a minhag: the practice of observing an extra day had developed for worthy religious reasons, and had become customary.
The word "minhag" is also used in a looser sense, to indicate a community or an individual's customary way of doing some religious thing. For example, it may be the minhag in one synagogue to stand while reciting a certain prayer, while in another synagogue it is the minhag to sit during that prayer. It may become an individual's minhag to sit in a certain location in synagogue, or to walk to synagogue in a certain way, and under appropriate circumstances these too may become minhag. Even in this looser sense, these customs can become binding on the individual, it is generally recommended that a person follow his own personal or community minhag as much as possible, even when visiting another community, unless that minhag would cause the other community discomfort or embarrassment.
As we have seen, Jewish law includes both laws that come directly from the Torah (either expressed, implied or deduced) and laws that were enacted by the rabbis. In a sense, however, even laws enacted by the rabbis can be considered derived from the Torah: the Torah gives certain people the authority to teach and to make judgments about the law (Deut. 17:11), so these rabbinical laws should not be casually dismissed as merely the "laws of man" (as opposed to the laws of G-d). Rabbinical laws are considered to be as binding as Torah laws, but there are differences in the way we apply laws that are "d'oraita" (from the Torah) and laws that are "d'rabbanan" (from the rabbis).
The first important difference is a matter of precedence: d'oraita takes precedence over d'rabbanan. If two d'oraita rules come into conflict in a particular situation, rules of precedence are applied to determine which rule is followed; however, if a d'oraita rule comes into conflict with a d'rabbanan rule, the d'oraita rule (Torah rule) always takes precedence. Do we fast on Yom Kippur when it falls on Shabbat? These are both d'oraita, so rules of precedence must apply. Specific rules take precedence over general rules, so the specific rules of Yom Kippur fasting takes precedence over the general rule of Shabbat joy, and yes, we fast on Yom Kippur on Shabbat. However, the other fasts on the Jewish calendar are d'rabbanan, so the d'oraita rule of Shabbat joy takes precedence, and other fasts that fall on Shabbat are moved to another day.
The second important difference is the strictness of observance. If there is doubt (in Hebrew: safek) in a matter that is d'oraita, we take the strict position (in Hebrew: machmir) regarding the rule; if there is doubt in a matter that is d'rabbanan, we take the lenient position (in Hebrew: makil) regarding the rule. In Hebrew, this rule is stated: safek d'oraita l'humra; safek d'rabbanan l'kula. This is easier to understand with an example: suppose you are reading the morning prayers and you can't remember whether you read Bar'khu and Shema (two important prayers). You are in doubt, safek. The recitation of Shema in the morning is a mitzvah d'oraita, a biblical commandment (Deut. 6:7), so you must be machmir, you must go back and recite Shema if you are not sure whether you did. The recitation of Bar'khu, on the other hand, is a mitzvah d'rabbanan, a rabbinic law, so you can be makil, you don't have to go back and recite it if you are not sure. If you are certain that you did not recite either of them, then you must go back and recite both, there is no doubt so no basis for leniency.