Eat the red pellets, and don't touch the walls or your own tail. Use the arrow keys to navigate. When the game is over, hit the space bar to set up a new game. While playing, you get one point each time you eat the last red pellet on the screen. If you take too long (15 seconds), two penalty pellets will appear, and you will get no score for eating them (although the snake will still grow).
Update 6/2/2007: Added sounds and a visible time left timer so that a player can see how much time remains before penaly pellets will show up.