A crossover is when a character from one story crosses over in the world of another story.
Television crossovers can happen for a number of reasons. A continuation of an earlier show: The Brady Bunch and then The Brady Brides. Or an engineered spin-off: Beverly Hills 90210 and Melrose Place. Or cross promotion: characters from established shows showing up on the show right before or after their own show: Friends, Mad About You, The Single Guy. Sometimes its subtle and jumps networks: Homicide Life on the Street and Chicago Hope.
Whatever the crossover is, you either love them -- like I do -- or think its nutty. I, for one, think its pretty cool that the world of Law And Order is in the same world as The X-Files.
You don't have to be a TV junkie (or someone with the ability to remember unimportant details and trivia) to know your TV crossovers. This amazing site, the Crossovers and Spin Offs Master Page, has it all. Not only does it mention specific crossover moments from shows; it then proceeds to link the shows that share the same reality. It includes all decades and all shows. You'll be the star of the reference desk with this unique source for all TV crossovers.