Chazal is an acronym for the Hebrew "Hachameinu Zichronam Lebracha", literally "Our Sages may their memory be blessed". In rabbinic writings this is a general term that refers to all sages of the Mishna, Talmud, and other rabbinic literature commentators, and their authoritative opinion, from the times of Second Temple of Jerusalem until the 6th century.

Chazal are generally being divided, according to their era and the main writing done in that era: (some eras overlap, years are approximate)

Sofrim (scribes): The most common rooted term used to all sages even before the era of Ezra HaSofer (the Scribe) and until the Zugot era, including the Men of the Great Assembly (Anshei Knesset HaGedola). This era stretches from the Matan Torah (giving of the Torah) event, on the 6th of Sivan 2448 (1312 BCE) (Talmud, Shabbat 86b), where Moshe Rabeinu (Moses) received the Torah on Mount Sinai, to the Halacha era, and until and including Shimon HaTzadik (the Righteous) times.

The Knesset HaGedola (Great Assembly) was a group of 120 sages headed by Ezra HaSofer. They included Mordechai (of the Purim story), Daniel (the lion’s den), Haggai, Zechariah, Malachi, Zerubavel, Nehemiah, Chanania, Mishael and Azaryah. Ezra HaSofer established this Great Assembly in Israel shortly after the beginning of the Second Jewish Commonwealth. "They said three things: Be deliberate in judgment, raise up many disciples, and make a fence around the Torah" (Pirkei Avot 1:1).

The Great Assembly was the Sanhedrin (Jewish religious law court) of its generation. The Sanhedrin had the authority to decree and to enact new religious regulations. These Rabbinical mitzvot include the holidays of Purim and Hanukah, the laws of Muktzah on Shabbat, the ritual washing of one's hands (netilat yadaim) before eating bread, the construction of eruvim, and the institution of the current schedule of daily prayer services (shacharit, mincha, and maariv). The Great Assembly also completed the total of the 24 books of the Tanach.

"Shimon HaTzadik was one of the last survivors of the Great Assembly. He used to say: On three things the world is sustained: on the Torah, on the (Temple) service, and on deeds of loving kindness." (Pirkei Avot 1:2)

Zugot: Pairs of Tannaim from consecutive generations, that lived in the midst of the Second Temple of Jerusalem era. There were five pairs of these teachers (Pirkei Avot 1:4-15):

1. Yosi ben Yoezer and Yosi ben Yochanan
2. Joshua ben Perachyah and Nittai the Arbelite
3. Judah ben Tabbai and Shimon ben Shetach
4. Shemayah and Avtalion
5. Hillel and Shammai

Hillel said: "What is hateful to you, do not do to your fellow. This is the whole Torah, the rest is commentary. Go and learn" (Talmud, Shabbat 31a). "If I am not for myself who will be for me? Yet, if I am for myself only, what am I? And if not now, when?" (Pirkei Avot 1:14). Shammai said: "Make your study of the Torah a fixed habit. Say little and do much, and receive all men with a cheerful face" (Pirkei Avot 1:15).

Tannaim: Refers to the sages of the Mishnah, who lived in the Land of Israel until the year 200 CE, and in addition to the Mishnah, their writings were also preserved in the Midrash. The Tannaim included Yochanan ben Zakai, Akiva ben Joseph, Rabbi Shimon bar Yochai, and Yehuda haNasi.

Yochanan ben Zakai was the youngest and most distinguished disciple of Hillel. He has been called the "father of wisdom and the father of generations (of scholars)" because he ensured the continuation of Jewish scholarship after Jerusalem fell to Rome in 70 CE. He established a school in Yavne which became the center of Jewish learning for centuries and replaced Jerusalem as the seat of the Sanhedrin.

Akiva ben Joseph (50-135 CE), a poor shepherd, became one of Judaism's greatest scholars. He developed the exegetical method of the Mishnah, linking each traditional practice to a basis in the biblical text, and systematized the material that later became the Mishnah. His favorite saying was "All that G-d does is for the good." Rabbi Akiva was active in the Bar Kochva rebellion against Rome, 132-135 CE. When the Bar Kochva rebellion failed, Rabbi Akiba was taken by the Roman authorities and tortured to death.

Rabbi Shimon bar Yochai, Rashbi, was one of the most eminent disciples of Rabbi Akiva, and is attributed by many with the authorship of the Zohar, the chief work of Kabbalah. In addition, the important legal commentaries called Sifrei and Mechilta are attributed to him. In the Mishna, he is often referred to as simply Rabbi Shimon. His son, Rabbi Eleazar ben Shimon was also a noted Kabbalist.

Yehuda HaNasi was the editor of the Mishna in its final form. He is referred to as "Rebbi," and as "Rabbeinu HaKadosh" (our Holy Rabbi). G-d gave both the Written Law (Torah she beKtav) and the Oral Law (Torah she beAlpeh) to Moshe on Mount Sinai. Fearing that the oral traditions might be forgotten, Yehuda HaNasi undertook the mission of compiling them in what became known as the Mishna. Rabbi Yehuda haNasi said: "What is the proper path a person should choose for himself? That which is an honor to him and elicits honor from his fellow men. Be careful with an easy mitzvah as with a hard one, for you do not know the reward for the mitzvot. Consider the loss incurred for not performing a mitzvah compared to its reward, and the reward received for sinning compared to the loss. Reflect on three things and you will never come to sin: Know what is above you: a seeing eye, a hearing ear, and all your deeds recorded in a book" (Pirkei Avot 2:1).

Amoraim: Refers to the sages of the Talmud, whom were active during the end of the era of the sealing of the Mishnah, and until the times of the sealing of the Talmud (from 200 CE and until 500 CE). Their legal discussions and debates were eventually codified in the Gemara. There are two versions of the Gemara, one compiled by the Amoraim sages of the Land of Israel and completed on 350 CE, and the other by those of Babylonia, primarily in the yeshivot of Nehardea, Sura and Pumbedita, completed 500 CE. In addition to the "Babylon Talmud" and the "Jerusalem Talmud", their writings were also preserved in the Midrashim, such as "Midrash Rabba".

There were approximately six generations of Babylonian Amoraim, whose discussions were compiled and edited by Rav Ashi and Ravina I in 420 CE. The final redaction of these discussions to form the Talmud was completed by Ravina II in 500 CE. The first generation of Amoraim was headed by Shmuel (in Nehardea) and Rav (in Sura). The second generation included Rav Kahana, and Rav Yehuda bar Yehezkel (in Pumbedita). The third, Rav Chisda, Rav Nachman, Rabba, and Rav Yosef. The fourth, Abaye, Rava, Rav Nachman. The fifth, Rav Pappa and Rav Hunna. The sixth, Rav Ashi and Ravina I (in Sura).

There were five generations of Amoraim in the Land of Israel. The first included Rav Yannai, Rav Yehuda, and Rav Yehoshua. The second, Rav Yochanan (in Tiberias), who was the driving force behind the Jerusalem Talmud, Rav Shimon ben Lakish, and Rav Elazar. The third, Rav Abbahu, Rav Ami, and Rav Assi. The fourth, Rav Avin, Rav Yirmeyah, and Hillel II (who systemized the Jewish calendar). The fifth generation included, Rav Yonah and Rav Tanchuma.

Savoraim: Refers to the sages in Babylon living from the end of the period of the Amoraim (500 CE) to the beginning of the Geonim (600 CE), who played a large role in editing and giving the Talmud its current structure. Up until the end of the Savoraim era, Chazal had the authority to commentate the Torah. Nowadays, the Torah cannot be commentated by the current generation's sages in matters concerning the Halacha, if it contradicts Chazal's commentary.

Geonim: The presidents of the two great yeshivot (rabbinical colleges) of Sura and Pumbedita, in Babylonia. The Geonim were recognized as the highest authorities in Jewish Law. Jews who lived even in distant countries sent their inquiries concerning religion and law to the Geonim in Babylonia. The Geonim played a prominent and decisive role in the transmission and teaching of Torah and Jewish Law. They taught Talmud and gave religious and legal decisions in agreement with its teachings. As the study of the Talmud received care in other lands, the Jews gradually began to submit their questions to the heads of their local yeshivot.

Rishonim: The leading rabbis and poskim (Jewish legal decisors) who lived approximately during the 11th to 15th centuries, in the era before the writing of the Shulchan Aruch and following the Geonim. The Rishonim include Rabbi Isaac ben Jacob Alfasi (Rif), Rabbi Shlomo Yitzhaki (Rashi), Rabbi Moshe ben Maimon (Rambam), Rabbi Moses ben Nachman (Ramban), Rabbi Asher ben Yehiel (Rosh), Rabbi Yaacov ben Asher (Baal HaTurim), and Rabbi Yosef Karo who wrote the Shulchan Aruch.

Acharonim: The leading rabbis and poskim living from roughly the 16th century to the present. The publication of the Shulchan Aruch thus marks the transition from the era of Rishonim to that of Acharonim. The Acharonim include Rabbi Eliyahu ben Shlomo Zalman (the Vilna Gaon), Rabbi Moshe Chaim Luzzatto, Rabbi Chaim Volozhin, Rabbi Israel Salanter, Ben Ish Chai, Chafetz Haim, Rabbi Nosson Zvi Finkel, Abraham Yitzchak HaCohen Kook, Chazon Ish, Rabbi Yosef Shlomo Kahaneman, Baba Sali, Rav Zvi Yehuda Kook, Rav Moshe Feinstein, Rav Elazar Menachem Man Shach, The Steipler, Rabbi Yitzchak Kaduri, Rabbi Yosef Shalom Elyashiv, Rabbi Aharon Yehuda Leib Shteinman, Rabbi Abraham Shapira, Rabbi Shlomo Goren, Rabbi Ovadia Yosef, Rav Ben Zion Abba Shaul, Rabbi Mordechai Eliyahu, and Rabbi Yisrael Meir Lau.

Scholars in one era within the history of halachic development cannot challenge the rulings of previous era scholars, unless they find support from other rabbis of previous eras.

In order to be able to fully appreciate who Chazal were, one needs to have a clear sense of what constitutes a Torah Scholar, or Chacham, also known as a Talmid Chacham. To achive the status of Talmid Chacham, one must satisfy several criteria. First and foremost is scholarship, he must have broad knowledge in all areas of Jewish studies (Shulchan Aruch, Yoreh Deah 243:2). Second, the Chacham is one who practices what he studies, there must be no incongruity between theory and observance (Talmud, Kidushin 40b). Third, the Chacham has refined character traits, a heightened moral sensitivity, and very high standards of interpersonal behavior (Rambam, Hilchot Deot 5).