Keriat Hatorah, or Leinen in Yiddish, is the public recitation in the synagogue of sections of the Five Books of Moses (The Pentateuch) which constitutes a very essential part of the service.
This practice is very ancient and the source is found in the Jerusalem Talmud (Megilla, 4) where it tells us that Moses established the custom of the Reading of the Law on Shabbat, festivals, new moons and the intermediate days of a festival (Chol Hamoed). Ezra the scribe, who led the return of Jewish exiles to the land of Israel in 450 B.C., extended the custom to include the morning services on Mondays and Thursdays.
The aim of this practice was to bring the law and its spirit close to the hearts of the people. Therefore on days when people were free from work (Shabbat, festivals etc.) or on Mondays and Thursdays which were market days in ancient times, people from the villages would gather in the nearest town where they would have the opportunity to go to the synagogue and listen to Keriat Hatorah. Later it was extended to the Mincha (afternoon) service on Shabbat, fast days (morning and afternoon services) and the festivals of Chanukah and Purim (morning service).
The custom nowadays is to complete the whole Torah in annual cycles. This begins on the first Shabbat after the festival of Succot (Tabernacles) which is called Shabbat Bereshit (after the name of the first portion read on that Shabbat) and ends on the last day of the festival of Succot the following year which is known as Simchat Torah (Rejoicing of the Law).
The Torah is divided into 54 portions called Parashot or Sidrot. When a festival occurs on a Shabbat, a special festival portion instead of the weekly portion is read. Therefore, because there are normally only 52 Shabbatot in a year, on certain Shabbatot, two Parashot are read.
The Torah is read with a special tune that consists of the ‘Teamim’. According to the Talmud (Megilla, 31a) one who reads the Torah without the proper tune shows disrespect to the Torah and disregard to its laws.