Most credit cards offer you a temporary loan: You get the card in the mail and you can charge whatever you want (up to your limit) until it's time to pay.
With pre-paid cards, you pay first. You put some cash on the card—say $500—and then you can spend up to that amount. You still get a bill every month and have to pay the $500; but the idea is that if you don't pay, the $500 is cooling its heels in the bank, and the bank can use it to pay off your plastic.
Why would anyone want a pre-paid credit card if you have to pay upfront? Well, if you have bad credit and can't get a traditional credit card, you can use a pre-paid card to build up your credit score.