The DC Universe (DCU) is the fictional shared universe where most stories in American comic book titles published by DC Comics take place. DC superheroes such as Superman, Batman, and Wonder Woman are from this universe, and it also contains well known supervillains such as the Lex Luthor, Joker, and Darkseid. In context, "DC Universe" usually refers to the main DC continuity.
The term "DC Multiverse" refers to the collection of all continuities within DC Comics publications. Within the Multiverse, the main DC Universe has gone by many names, but in recent years has been referred to by "Prime Earth" (not to be confused with "Earth Prime") or "Earth 0".
The main DC Universe as well as the alternate realities related to it began as the first shared universe in comic books and were quickly adapted to other media such as film serials or radio dramas. In subsequent decades, the continuity between all of these media became increasingly complex with certain storylines and events designed to simplify or streamline the more confusing aspects of characters' histories.
Crisis on Infinite Earths
The New 52
Heaven and Hell
In other media