It's a site where you do things and you receive "PTZ", which is a type of currency. After you accumulated enough PTZ, you can exchange with items like ipods, video games, etc (Which was mostly "Fresh Out" last time I was on). If you invited 20 people, then you will get double PTZ. For example, you get 2 PTZ for logging in, after the 20 invite, you will get 4 PTZ from now on.
Some people think this is a type of pyramid scheme. I tried it, and I stopped using it after a month or so.