You are in control of an artificially intelligent tank, that along with its fellow AI tanks, has decided to abandon its human masters and just go have some fun.
The tanks come in two varieties: steel and liquid. The steel tanks resemble real-life tanks, while the liquid types are whimsical fantasy tanks. Weapons and power-ups are scattered around the maps, and range in destructive power from light machine guns to tactical nuclear missiles.
Source: Wikipedia, "Tread Marks", available under the CC-BY-SA License.