A crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders (known as intercompany crossovers), common corporate ownership or unofficial efforts by fans.
This is different from a spoof, where one discrete character, setting, or universe, copies another character, setting, or universe, often in a comedic manner.
Background
Official
Crossovers often occur in an official capacity in order for the intellectual property rights holders to reap the financial reward of combining two or more popular, established properties. In other cases, the crossover can serve to introduce a new concept derivative of an older one. Another intention is to give fictional characters more emotional credibility and thus increase immersion for the fans.
Crossovers generally occur between properties owned by a single holder, but they can, more rarely, involve properties from different holders, provided that the inherent legal obstacles can be overcome. They may also involve using characters that have passed into the public domain with those concurrently under copyright protection.
A crossover story may try to explain its own reason for the crossover, such as characters being neighbors (notable examples being the casts from The Golden Girls and Empty Nest) or meeting via dimensional rift or similar phenomenon (a common explanation for science fiction properties that have different owners). Some crossovers are not explained at all. Others are absurd or simply impossible within the fictional setting, and have to be ignored by the series' respective continuities. Still others intentionally make the relations between two or more fictional universes confusing, as with The Simpsons and Futurama, where each show is fiction in the other.
Unofficial
In contrast with legal crossovers, unofficial crossovers are created solely because of the artistic pleasure derived by their creators. Unofficial crossovers often take the form of fan-written fiction and fan art, but the trope is increasingly prevalent in amateur films and audio. Whereas official crossovers are frequently stymied by such concerns as copyright, royalties payments, quality of writing and ownership of the characters, unofficial crossovers are unfettered by such concerns, so long as property holders do not exercise their right to enjoin the distribution of such material. A good example would be the unauthorised live action fan film Batman: Dead End which brings together the properties of Batman, Alien and Predator in one setting.
Unofficial crossovers can also occur in a "what-if" scenario. Roger makes frequent cameo appearances in Family Guy, while Brian makes cameos on American Dad!. Roger, Rallo Tubbs, and Klaus Heissler were seen in the final Family Guy Star Wars spoof, "It's A Trap!", as Moff Jerjerrod, Nien Nunb, and Admiral Ackbar, respectively. Stewie also appears as an interactive hallucination of Booth on Bones when the agent has issues over possibly becoming a sperm donor, with David Boreanaz (who plays Booth) repaying the favor in "Road to the North Pole". An appearance by Elmo, from Sesame Street, was made, in a hallucination of Connie Ray's, on TV sitcom The Torkelsons. Fan fiction fusions between different science fiction movies and series are often created, such as Star Wars and Star Trek or Babylon 5 and Stargate. M.U.G.E.N. is a fighting game engine that features many fan-created and fictional characters and stages from various television series, movies, as well as other video games.
Comics
Crossovers of multiple characters, owned by one company or published by one publisher, have been used to set an established continuity, where characters can frequently meet within one setting. This is especially true of comic book publishers, as different characters in various Marvel, DC, or Valiant comic books frequently interact with one another since they live in a "shared universe". For example, in the Marvel Comics universe, Spider-Man has frequent dealings with another Marvel hero, Daredevil, just as in the DC Comics Universe, the Flash and Green Lantern often collaborate. In comic book terminology, these "guest star" roles are common enough that they are generally not considered crossovers; rather, this short-term collaboration to fight crime is called a team-up. A crossover in comic book terms only occurs when a story spans more than one title. This has led to "crossover events" in which major occurrences are shown as affecting most or all of the stories in the shared universe; see Category:Crossover comics.