Looney Labs claims to have sold more than 350,000 copies of Fluxx, which is a truly remarkable number for a hobby game from a tiny publisher -- remarkable also because Fluxx is in some ways a game designer's game, exploring the nature of "the rule."
In a sense, Fluxx is a stripped down version of Nomic -- a self-modifying game with a small initial set of rules and the ability to change them. Nomic depends on player voting; Fluxx depends on card-play.
Fluxx can also be thought of as a form of what Richard Garfield calls an "exceptions game" -- a game that, like Magic or Cosmic Encounter, has a small base rules set that is modified and complexified by additional rules printed on components that may or may not come into play over time. (The additional rules are "exceptions" to the original rules; the virtue of this style of game is that learning the base rules is quick, and additional rules need be learned only as they appear, thus allowing a game that appears simple but actually possesses considerable depth and complexity.)
Here's Fluxx's base rules: Draw one, play one. Okay, it's a bit more complicated than that; you shuffle the cards, deal three to each player, and when it is your turn, you draw a card from the deck and play one. Initially, there is no win condition.
Some cards are "keepers," which you play to the table in front of you and keep. Keepers have names (e.g., "Bread"), but mean nothing in themselves--but if a Goal card has been played, it stipulates a win condition (e.g., "the player who has both Bread and Chocolate wins"). Naturally, only one Goal is in effect at any time, but a player can change the game's Goal by playing a new Goal card.
Similarly, rules cards can be played to change the rules--increasing or decreasing the number of cards drawn and/or played each turn, and establishing a "hand limit" (e.g., a hand limit of 3 means you must discard any cards you possess above three at the end of your turn). Other cards are "instants," meaning you play them and follow the instructions, after which they are discarded--typically, these might allow you to take a keeper from another player or the like.
The charm of Fluxx lies partly in its Nomic conventions, and partly in the game's instability -- the play of a single card can alter its dynamics drastically. It is, in some ways, a flawed game; its instability also means that it's virtually impossible to conceive and execute a strategy, since things change unpredictably and frequently. Yet it's worth playing to experience its unique and original design.