Fluxx is a simple card game with rapidly changing rules; at the beginning of the game, the rules just say 'draw one card, play one card', so that's what you do. As the game progresses, these rules will change, and you may even find out how to win!
The cards are of four types;
The goals and keepers have clearly been chosen by someone with a sense of humour, and the play of the cards is fast, fun, and addictive, with little thought required. A good game to play with the family, the kids, or at the end of a gaming session (don't bring it out too soon - you may not want to play anything else!).
For those who want a little more skill, Steffan O'Sullivan has a few suggestions about the game on his WWW site.