chitas (Hebrew: חת"ת) is a Hebrew acronym for Chumash (the five books of Moses), Tehillim (Psalms), and Tanya (a seminal work of Hasidic philosophy by Rabbi Schneur Zalman of Liadi, the Alter Rebbe). These are considered basic Jewish texts according to the Chabad tradition, and so there is a custom to study these works according to a yearly cycle, which is known colloquially as "doing ChiTaS."
The divisions of the Chumash and the Psalms are many centuries old, but the Tanya was divided into daily study portions by Yosef Yitzchok Schneersohn.
Rabbi Menachem Mendel Schneerson often encouraged Jews to follow this cycle, emphasizing that these study portions are applicable to every single Jew.
These three texts have been bound together in one volume, which is available from the Kehot Publication Society. The volume also includes other elements of daily use, such as the Siddur.
Beginning in 1942, Chabad Chassidim study in addition the thought for the day from Rabbi Menachem Mendel Schneerson's timeless calendar, the Hayom Yom.
In 1984, Rabbi Menachem Mendel Schneerson also instituted the study of Maimonides' seminal work the Mishneh Torah, to be completed in a cycle lasting just under a year.