The DC Universe, or DCU, is a single phrase used for three distinct meanings. These are: the entire fictional multiverse in all comic books owned and published by DC Comics exist, the primary parallel reality featured in most DC comics, and a broad category used to define all comic books set within the primary parallel reality.
The nature of the DC multiverse has changed several times. In its original form, the DC Universe comprised a limitless number of parallel universes, offering storytellers endless possibilities to explore alternative takes on characters (called Elseworlds) and allowing characters to travel from one strange universe to the next.
However, the end result of this was that some comics permanently existed in parallel realities vastly different from other comics on the same shelf. Other comics, meanwhile, regularly featured parallel universe versions of characters that had died in the main universe. Worried that this would be confusing to new readers, the editors created a super-arc, The Crisis on Infinite Earths, which spanned almost all of the DC Universe line. It climaxed with all the universes colliding, forming a single new universe with a single continuity. A similar event occurred in 1994's Zero Hour.
In 2006, DC decided to restore the multiple-universe status quo in its event Infinite Crisis. This ended in the creation of 52 parallel universes, each subtly different from the other.
The primary universe
The main DC Universe is different from the Vertigo Universe, which was established in 1993 to contain DC-owned mature readers comics, thus keeping the likes of John Constantine away from younger-reader-friendly characters such as Superman. The differences between these two became blurred in 2008, when the story Infinite Crisis culminated in the creation of 52 parallel universes existing within the DC multiverse - one of these universes, it was revealed, being the Vertigo universe.
The line was further blurred in 2011, when DC versions of John Constantine and Swamp Thing were introduced into the main DC Universe, to interact with the likes of Superman, Batman and Wonder Woman as the writers see fit.
All comic books taking place in the primary parallel reality are said to be DC Universe books, and share a single (if not always consistent) continuity. Stories set in alternate universes may also be called DC Universe books for ease of understanding - the only exception to this rule being Vertigo comics.
Notable and relevant comics
The following Constantine-related comics are based in the DCU: