PPC stands for "pay per click". Pay per Click is an advertising model where businesses pay an agreed upon amount, each time their advertisement is "clicked", not each time their ad is displayed.
Advertisers typically setup ppc on a PPC network and define how much they are willing to spend for each click-through that they receive.
Advertisers select keywords, keyword phrases, keyword groups, or categories in which they want their advertisements to appear.
Advertisers willing to spend the most money for a relevant advertisement will generally be listed first. *Google's PPC ranking system assesses CPC (cost per click), ad relevance, click through rate, and daily budget, so the order on which the ads appear is based on a number of factors.