Crossover

A crossover is a fanfiction that combines two or more fandoms. They range from the almost likely to the completely implausible. Crossovers of the implausible variety are dealt with by the Department of Implausible Crossovers (DIC), which is headed by the Lichen. Real World crossovers are the purview of Despatch.

Standard Crossovers
The most common kind, a standard crossover occurs when a fanfic author smashes together two continua that are not canonically connected. This can be done very successfully, or it can leave the reader going "WTF?" The Harry Potter continuum has very many standard crossovers. It is a generally acknowledged rule that if a fandom exists, it has been crossed with Harry Potter.

These crossovers will usually include a plot device to make interaction between the continua possible. This plot device can be good, bad, or sometimes just dumb.

Real World Crossovers
Continua can be uncanonically connected to the Real World by virtue of a character from one world traveling to another by means of trans-dimensional hopping or snatching. Technically these stories are a type of crossover, but the contact between the two worlds is usually limited.

Very often, the main character from the 'real world' half of the crossover is a self insert or an avatar. These character types are very prone to becoming Mary Sues, if they fail to act, think, and grow as a real person would.

In the case of Trans-Dimensional Hopping, the crossover is done by an OC traveling from the Real World to a canon world of his or her choice.

In the case of Trans-Dimensional Snatching, an authorial influence or Mary Sue transports canon characters from their home continuum to the Real World. "BagEnders" is an example of this type of story done well. Unfortunately, what often happens is an insipidly cute sleepover at the writer's house, or all the members of the Fellowship becoming stereotypical teenagers at the same high school.

Actor Crossovers
An actor crossover typically involves a character meeting another character played by the same actor. This type often results from the same actor playing parts on two popular television shows or movies. (For example, David Tennant in Doctor Who and Harry Potter and the Goblet of Fire, or Derek Jacobi in The Golden Compass and I, Claudius.) There are usually many bad jokes about how the two characters look like each other.

Another type of actor crossover is a variety of Real Person Fic in which an actor enters a canon continuum and meets the character he plays. This type is less common, but also involves bad jokes.

Blender Crossovers
According to Architeuthis, in this type of crossover, "different canonical characters from different continuums [sic] are 'revealed' to be actually the same person." For instance, "Well, Gandalf and Dumbledore are both magic-users with long beards, so... wow, they must be the same person!"

This may happen with settings as well as people. It is very common for people to decide that Harry Potter and Yu-Gi-Oh!, for example, have the same planet earth as a setting. Or to write about Star Wars and Star Trek taking place in the same universe. Goodfic that invokes this meshes the worlds sensibly and interestingly. Badfic that invokes this kind of crossover leaves gaping plotholes trying to explain away where incompatable plots in two different works of fiction would otherwise make eachother impossible-- or even worse, doesn't even try to make sense of it at all.

Canon Crossovers
This rare type is a crossover that takes place in the actual canon. It usually occurs when two or more television shows are owned by the same network or when a show has several spin-offs. The Disney Channel is rife with canon crossovers.