Goat Simulator was a game inspired by old-school skating, but for testing, the developpers instead used a goat. This proved to be popular among testers, and the concept of a goat, and all the graphic bugs and various hilarities, were kept for the final release. The game relies on being a completely whacky, silly and half buggy physics engine environment.
The game is a virtual playground, when you run around and normally wreak havoc by headbutting people and objects. The goat can lick things, which will then stick to its tongue, which in turn is highly elastic and can stretch out for tens of meters. The goat can via the menu be equipped with various "mutators", such as double jump, high speed head butting or others. The two stages have a number of "trophies" that, when collected, will unlock more mutators. The stages also contain more or less hidden objectives that, when found, will unlock even more mutators. There is no specific goal of the game; the whole idea is just to run around and find weird things to do, such as licking a roller coaster wagon, or maybe buy a new hat for your goat.