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?" 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 does this meshes the worlds sensibly and interestingly. Badfic that does this leaves gaping plotholes trying to explain away where incompatible plots in two different works of fiction would otherwise make each other impossible&mdash;or even worse, doesn't even try to make sense of it at all.

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.

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.

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. When this is done well, you get something like "Don't Panic!" by Boz4PM, which explores the logical outcome of a modern woman dropping into an ancient world where she doesn't even speak the language. When this is not done well... see Mary Sue.

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!"

When the same principle is applied to canon settings, you get a standard crossover.

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.

Links
Lily's Tutorial on Fandom Collision NSFW. A illustrated guide by Lily Winterwood on how to distinguish the most common types of crossovers and how to avoid the pitfalls of writing them badly (hint: know your fandoms). It's easy to read, easy to print and easy to bring on missions, if you are new to crossovers.