This is the process of paying for traffic with networks like AdWords or Microsoft AdCenter with the hopes of converting that traffic into sales.
Say you are selling iPhone cases and you want your site to be ranked in Google's Sponsored Listings when someone searches for "iPhone cases." You would bid on a ranking with other advertisers, and your final rank depends on the overall quality/relevancy of your site and your maximum bid.
This is a great way to make a lot of money selling your own products and through affiliate marketing, but there is a LOT of trial and error involved here. Most successful PPCers lose a lot of money before they make a profit.
