A "crossover" is when characters from one series/property appear in an unrelated series/property. In comic books, the "shared universe" idea is so intrinsic, characters regularly guest star in each other's magazines. There are different reasons for doing a TV crossover. The first is just for the fun of it.
A recurring gripe of mine is how many Canadian movies and TV shows slavishly pretend they aren't Canadian -- such as by disguising Toronto as New York or as a generic "Anytown, North America." So how to best test this idea...and have fun at the same time? Let's make a drinking game out of Canadian references!