Look at those happy people dancing. It's because they love their goat friend. Unfortunately, their goat friend murdered them and blew up everything in their town.
The aim of Goat Simulator is to get points for destroying things by headbutting them. There are achievements for everything from jumping a certain height to an achievement that just says "MICHAEL BAY".
The game is also hilariously broken, but in a good way. From the game's site:
Goat Simulator is a small, broken and stupid game. I t was made in a couple of weeks so don't expect a game in the size and scope of GTA with goats. In fact, you're better off not expecting anything at all actually. To be completely honest, it would be best if you'd spend your $10 on a hula hoop, a pile of bricks, or maybe a real-life goat.
So if you're looking for a polished narrative experience about the life of a goat, this isn't the game for you. But if you've got a pc and you're looking for a game about a goat that can headbutt a gas station into oblivion, then head over to the game's site and pick it up for $10.