What is pay per click?
Pay Per Click adverts are known as "sponsored links" on Google, "sponsor results" on Yahoo, and "sponsored sites" on Microsoft's Live - the world's 3 major search engines.
The general concept of PPC advertising is based on 2 elements:
1) Contextual Advertising
Showing a relevant ad (i.e. an ad that is "in context") at the time someone is searching for / reading about a particular subject.
2) Payment on Results
You only "pay per click", that is, each time someone clicks on your ad to visit your web site, rather than is simply exposed to it, as per conventional advertising.