A small cake (or cookie) served at the end of a multi-course meal or elaborate buffet. They're usually about 1" square and traditionally come in two varieties: Secs (dry) and Glacé (iced). The dry kind include dainty cookies, meringues, macaroons and puff pastries. The iced include little cakes covered in fondant, small eclairs and tartlets.
Oh and my favorite part, the name "petit four" means "small oven" in French.