Set in the same universe as the Eddings' The Belgariad and The Malloreon, it is effectively one big flashback (or a prequel) to the other series, although it is technically set after the events of The Malloreon.
The book opens shortly after the end of The Malloreon with Belgarion pestering Belgarath to write an autobiographical account of the events prior to The Belgariad. The core of the book is in the form of Belgarath's memoirs starting with his becoming an outcast from his village and becoming first disciple of the god Aldur and ending with the birth of Belgarion—a span of about seven thousand years.
Belgarath the Sorcerer is a prequel to Polgara the Sorceress which form the memoirs of Belgarath's elder daughter Polgara.