What is Cosplay?

Cosplay is basically just dressing up as a character from a series/movie/book/etc. Sometimes it includes roleplaying as the character, but most often I've seen people just hanging out dressed up as characters they like. It's popular at various types of conventions, late-night showings of movies, and any type of fan event. Most of the cosplaying I've seen has been involved with anime, and has happened at anime conventions. It can get pretty out-there, with people spending a lot of time and money on costumes, and often there are competitions.