Rewards credit cards tend to have annual fees, so they really aren't worth getting unless you plan to use it enough to get a substantial amount of rewards.
When you make a purchase using the card you received a small percentage of the money spent back on the card. An example could be 5% of your total purchase.
You recieve rewards or points by using the credit card. You just need to make sure that you pay the balance every month or the interest charged outweighs the rewards.