
It was an "experiment" by a man called Ian Bogost. I believe the way it works is:
1. You can only click the cow once a day
2. It shares on Facebook "xxxx has Clicked a Cow"
3. If you want to click the cow again that day, you have to pay money.
That's it.
( , Thu 23 Aug 2012, 9:38, Reply)

he made it as a joke, taking the piss out of that sort of game. And it became the most commercially successful thing he had ever done.
( , Thu 23 Aug 2012, 12:19, Reply)