A shared universe is a setting in which multiple writers create numerous characters who are all able to interact, and where events in one story can impact another. Some of the most common examples of this type of work are comic book series created by numerous authors and artists, involving multiple characters in their own titles, that are all published by a single company. Many times, those characters can interact with each other, appearing in the books of other characters, and participate in massive "cross-over" events. There are also literary examples of writers working together in a shared universe, such as the "Cthulhu Mythos" and the Expanded Universe (EU) of Star Wars®.
While similar to a collaborative work, a shared universe typically has certain elements that differentiate the two concepts. Collaborative works are often one or more books that are written by multiple authors. There is a single story being told throughout and the characters remain consistent, with each writer contributing different sections to the story and creating the overall narrative. In a shared universe, however, multiple books and short stories can be written by different authors, each often having its own cast of characters, and the events in one story impact another.
For example, many comic book publishers have numerous titles, each one focusing on a different character, often a super hero. Some publishers, such as Marvel® and DC®, have their characters occupy a shared universe, so that each title is taking place in the same world as every other. This allows characters from one title to show up in another; these events are often referred to as "cross-overs" and can be major story lines within the continuity of all comics from one publisher. In this type of shared universe, the writer of one character can borrow villains or heroes from another title, and events that take place in one series can be used to alter the narrative of another story.
One of the most famous examples of a shared universe in literature is the "Cthulhu Mythos," which has been built upon the writing of Howard Phillips Lovecraft. Many of Lovecraft's works are horror stories, or part of a genre known as strange fiction, his tales were often about fear and paranoia due to unknowable horrors from outer space. His stories influenced other writers, who created similar tales and even borrowed characters and creatures from his works, which he ultimately encouraged and worked with some of these writers in his lifetime. Since Lovecraft's death, more authors have created their own stories that are all part of the same shared universe, which has become known as the "Cthulhu Mythos," named for the creature that is the central figure in one of Lovecraft's stories, Call of Cthulhu.