Depending on the card, you get around 1 or 2 or even 3% back in cash or other types of rewards like airline miles on your purchase. Basically the credit card company charges a merchant something like 3% on a transaction they accept through a credit card, and the credit card company decides to pass on a portion of that fee to the credit card user as an incentive to use that credit card over another form of payment.

In my case, I pay for as much as I possibly can using a credit card and then pay it off at the end of the month so that I can get that % reward back and am not charged any fees.