There's a number of alternatives to accepting credit card payments directly on your site. Some of these include:
Paypal
Google Checkout
2Checkout.com
There's some advantages and disadvantages to linking customers to these external sources rather than accepting credit cards directly on your site.
Advantages
Easy to set up
No setup or monthly fees
You don't need SSL on your web server
Payment is being handled by a trusted third party
Disadvantages
They take a higher percentage of your sales than most
credit card processing providers
It makes your site look somewhat unprofessional
You loose some visitor tracking capabilities
In most cases the best solution is to offer both direct credit card processing from your site, and one or more of these third parties as an alternative payment method.