The DC Universe (DCU) is the shared universe where most of the comic stories published by DC Comics take place. The fictional characters Superman, Batman, and Wonder Woman are well-known superhero from this universe. Note that in context, "DC Universe" is usually used to refer to the main DC continuity. Occasionally, "DC Universe" will be used to indicate the entire "DC Multiverse", the collection of all continuities within DC Comics publications.
The concept of a shared universe was originally pioneered by DC Comics (originally known as National Periodical Publications) and in particular by writer Gardner Fox. The fact that DC Comics characters coexisted in the same world was first established in All Star Comics #3 (1940) where several superheroes (who starred in separate stories in the series up to that point) met each other, and soon founded the superhero team, the Justice Society of America. However, the majority of National/DC's publications continued to be written with little regard of maintaining continuity with each other for the first few decades.
Over the course of its publishing history, DC has introduced different versions of its characters, sometimes presenting them as if the earlier version had never existed. For example, they introduced new versions of the Flash, Green Lantern, and Hawkman in the late 1950s, with similar powers but different names and personal histories. Similarly, they had characters such as Batman whose early adventures set in the 1940s could not easily be reconciled with stories featuring a still-youthful man in the 1970s. To explain this, they introduced the idea of the multiverse in Flash #123 (1961) where the Silver Age Flash met his Golden Age counterpart. In addition to allowing the conflicting stories to "co-exist," it allowed the differing versions of characters to meet, and even team up to combat cross-universe threats. The writers gave designations such as "Earth-One", "Earth-Two", and so forth, to certain universes, designations which at times were also used by the characters themselves. Earth-One was the primary world of this publication era.
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Official Site: DC Comics
Links: Wikipedia DC wiki Chronology of the DC Universe