You don't actually have to have a paypal account to pay with paypal. You can just pay through them using your regular credit card, and it's just like any other cc transaction at any store. And you'll have the advantage of having the protections afforded by using your credit card.
To pay without having a paypal account, you just need to go to the paypal.com home page and click on "send money." You'll be given the option to pay with a credit card, or pay with a paypal account.
Paypal, in addition to their regular account services, is one of the largest merchant services providers (credit card gateways) around. Many companies, large and small use them for their credit card handling. If you're shopping at a web store and have a choice between using paypal or a credit card, lots of times your transaction will go through paypal either way.