Kill bugs and other players in an effort to become the richest (and cutest) animal in the game. Progress through 12 different weapons, and 10 different characters, and bank money to save it for later.
Just move with WASD and use the mouse to aim and shoot.
You can pick up weapons dropped by other players to get a feel for many of them before unlocking them yourself. It's a great way to find one that clicks with you and to focus on unlocking it.