You're monsters squabbling over control of tokyo. There are six 6 sided dice with the following results: attack, health, energy, 1, 2, and 3. If you can roll triples on any number you get that many victory points, and need 20 vp to win (i.e. if you roll three 1's you get 1 VP). You heal 1 damage for every health result, and do 1 damage for every attack result (all monsters have 10 hit points). Each energy result gives you 1 energy, which can be spent to buy Keep cards. Keep cards represent monster powers and abilities, or stuff you blow up in the city for VP.
Everyone rolls all 6 dice and the one with the most attack results goes first. The first monster to roll an attack result after that goes to tokyo, and gains 1 vp (you gain 1 vp when you enter tokyo, and 2 more when you begin your turn there). Once a monster is in tokyo, any attack rolls he makes are against all monsters not in tokyo, and all monsters outside of tokyo can only attack him. So when you're in Tokyo you do a ton off damage to rivals, and gain lots of VP, but you cant heal. So if you get too hurt you can voluntarily drop out of tokyo, and the monster who attacked you takes your place.
The first expansion has a new monster and Evolution cards. Evolution cards are monster specific cards representing the powers of that critter, and allowing further customization. If you roll 3 health you draw 1 Evolution, so it alters strategy a little.
The second Halloween themed expansion adds two monsters, and costumes. You can now be Princess Godzilla.
There;s also a variation of the game thats slightly more complex called King of New York. Iello has the rules for both games posted on their site.