It depends on which one you choose. Most people seem to favour the Cheetah and Gazelle one.
So, you either spawn in the map as a gazelle or a cheetah, and you basically have to survive for as long as you can.
As a gazelle you must eat, drink, fight other gazelles for females (your always a male gazelle) and try to not get killed by the cheetah.
As a cheetah, you must hunt the gazelles, kill them, eat them, drink and survive for as long as possible.
It's also a rotational thing, so at the end of each session everything will go back to the start and you might turn up as a gazelle again or you might be the cheetah.
It's quite fun...up until people start getting annoyingly competitive.
I'm not sure xD Perhaps they get frustrated.