Number and time
There are three prayer services each day on weekdays. A fourth additional prayer service (called mussaf, "additional"), is added on Shabbat (the Jewish Sabbath) and on major holidays by Orthodox and Conservative congregations. A fifth prayer (ne'ilah), is only recited on Yom Kippur.

According to the Talmud (tractate Taanit 2a), prayer is a Biblical command: "You shall serve God with your whole heart (Deuteronomy 11:13) - What service is performed with the heart? This is prayer". The prayers are therefore referred to as Avodah sheba-Lev (service from in the heart). Maimonides (Mishneh Torah, Laws of Prayer 1:1) likewise categorises prayer as a Biblical command, but states that the number of prayers or their times are not. This statement is relied upon by the authorities that hold that women, while being required to pray, only need to pray once a day (preferably in the morning), though they can, if they wish, pray all three daily prayers.

The Talmud (tractate Berachoth 26b) gives different reasons why there are three basic prayers.

According to the standard view held by Orthodox Judaism, each service was instituted parallel to a sacrificial act in the Temple in Jerusalem: the morning Tamid offering in the morning for the morning, the afternoon Tamid for the afternoon prayers and the overnight burning of the leftovers for the evening prayers.
According to one sage, however, each of the Patriarchs instituted one prayer: Abraham the morning, Isaac the afternoon and Jacob the evening prayers. This view is supported with Biblical quotes indicating that the Patriarchs prayed at the time mentioned.
Additional Biblical references suggest that King David and the prophet Daniel prayed three times a day. In Psalms, David states: "Evening, morning and afternoon do I pray and cry, and He will hear my voice" (55:18). As in Daniel: "[...] his windows being open in his chamber toward Jerusalem, he kneeled upon his knees three times a day, and prayed, and gave thanks before his God, as he had done before" (6:11).

Orthodox Judaism regards halakha (Jewish law), as requiring Jewish men to pray three times daily and four times daily on the Sabbath and most Jewish holidays, and five times on Yom Kippur. Orthodox Jewish women are required to pray at least daily, with no specific time requirement, but the system of multiple daily prayer services is regarded as optional. Conservative Judaism also regards the halakhic system of multiple daily services as mandatory. Since 2002, Conservative Jewish women have been regarded as having undertaken a communal obligation to pray the same prayers at the same times as men, with traditionalist communities and individual women permitted to opt out.. Reform and Reconstructionist congregations do not regard halakha as binding and hence regard appropriate prayer times as matters of personal spiritual decision rather than a matter of religious requirement.

Text and language
Maimonides (Mishneh Torah, Laws of Prayer 1:4) relates that until the Babylonian exile, all Jews composed their own prayers. After the exile, however, the sages of the time (united in the Great Assembly) found the ability of the people insufficient to continue the practice, and they composed the main portions of the siddur, such as the Amidah. The language of the prayers, while clearly being from the Second Temple period, often employs Biblical idiom, and according to some authorities it should not contain rabbinic or Mishnaic idiom apart from in the sections of Mishnah that are featured.

Over the last two thousand years, the various streams of Jews have resulted in small variations in the traditional liturgy customs among different Jewish communities, with the each community having a slightly different Nusach (customary liturgy). The principle difference is between Ashkenazic and Sephardic customs, although there are other communities (e.g. Yemenite Jews), and Hassidic and other communities also have distinct customs, variations, and special prayers. The differences are quite minor compared with the commonalities.

According to halakha, all individual prayers, and virtually all communal prayers, may if desired be said in any language that the person praying understands. Nonetheless the tradition of most Ashkenazi Orthodox synagogues is to use Hebrew (usually Ashkenazi Hebrew) for everything except for a small number of prayers, including the Kaddish, which had always been in Aramaic, and sermons and directions, for which the local language is used. In other streams there is considerable variability. Sephardic Orthodox communities may use Ladino or Portuguese for many prayers. Conservative synagogues tend to use the local language use in at least some prayers, while at some Reform synagogues almost the whole service may be in the local language.

Conservative services generally use the same basic format for services as in Orthodox Judaism with some doctininal liberalizations and some prayers in English. In practice there is wide variation among Conservative congregations. In traditionalist congregations the liturgy can be almost identical to that of Orthodox Judaism, almost entirely in Hebrew (and Aramaic), with a few minor exceptions, including excision of a study session on Temple sacrifices, and modifications of prayers for the restoration of the sacrificial system. In more liberal Conservative synagogues there are greater changes to the service, with 20% to 50% of the service in English, abbreviation or ommission of many of the preparatory prayers, and the replacement of some traditional prayers with more contemporary forms. There are often also additional changes for doctrinal reasons, including more egalitarian language, additional excisions of references to the Temple in Jerusalem and sacrifices, elimination of special roles for Kohanim and Levites, etc.

Reform and Reconstructionist use a format which is based on traditional elements, but contains language more reflective of liberal belief than the traditional liturgy. Doctrinal revisions which may vary from congregation to congregation but generally include revising or omitting references to traditional doctrines such as bodily resurrection, a personal Jewish Messiah, and other elements of traditional Jewish eschatology, Divine revelation of the Torah at Mount Sinai, angels, conceptions of reward and punsihment, and other personal miraculous and supernatural elements. Services are often mostly in English, with English content varying from 40% to 90%.

Quorum
Individual prayer is considered acceptable, but prayer with a quorum of ten adults (a minyan) is considered "prayer with the community", and this is the most highly recommended form of prayer. Judaism has traditionally counted only men in the minyan for formal prayer, on the basis that one does not count someone who is not obligated to participate. Since 1973, many Conservative congregations have begun to count women in the minyan as well, although the determination of whether or not to do so is left to the individual congregation. Those Reform and Reconstructionist congregations that consider a minyan mandatory for communal prayer, count both men and women for a minyan. In Orthodox Judaism, according to some authorities, women can count in the minyan for certain specific prayers, such as the Birchot HaGomel blessing, which both men and women are obligated to say publicly.