With a postpaid plan, you pay for service at the end of a specified term of use. Generally this includes a flat monthly fee, plus added charges for additional minutes of usage. With a prepaid contract, you pay for service in advance. For instance, if you purchase 20 hours of talk time, your service will work for 20 hours only.
Prepaid plans offer the convenience of only paying for the minutes you actually use, but postpaid plans are generally considerably cheaper in the long term and include perks like free phones, free long distance and discounted upgrades.