You can read a bit more about Sweepstakes and how to work with them on LosPollos blog.
Here is a short glimpse:
"The actual definition of Sweepstakes is “lottery”, a form of gambling or a promotional drawing in which prizes are given away at no charge to the participants. When it comes to Affiliate Marketing, it is an online draw on social networks with a random selection of the winner. To become a member and have a chance to win the coveted prize, users must provide information about themselves – at a minimum, a name, and email address. Most often, sweepstake offers are found on social networks with a drawing of the latest iPhone, a discount coupon or certificate, household appliances, or, even, a car."
Their article might give you a better idea as to what and how.