have you ever heard of the game "Worms" its basically the same idea.
The very basic level, you take turns shooting at each other until they fall to their deaths or u kill all of their hit points.
http://www.gunbound.ijji.com
To add variety there is some weather effects that change the power of your shots or how they travel. Than there are also items to make your shot achieve different effects.
There are avatars to change the look of your avatar and soem add bonus stats that only work in "Avatar On" server.
I guess that is the basics.