A Pay-Per-Click search engine enables you to list your site at the top of the search engine results for a particular keyword if you are willing to pay (through an auction process) for each time someone clicks on that listing.
The Pay-Per-Click listings are usually marked as different from non-paid search listings (e.g. Sponsored Links). There are usually 3-4 listing spots available at the top of a page for a given keyword.
Each keyword is sold separately and your advertisement's position, and frequency of display are influenced by how much you are willing to pay per click.
In addition, some search engines display advertisements based on the relevance of the advertisement to the keyword being bid on.