A crossover is a comic where you take in characters from another comic.

For example: The smurfs are coming to Duckburg. The old smurf (the one with the red hat) and Gyro Gearloose sit together discussing inventions and chemistry while the female smurf teaches Daisy to use less make-up more effeciently.

So, you take a comic you like and combine it with characters from other comics you like.