PPC engines work on the premise that you 'pay per click.' These programs can have up to millions of publishers displaying their content or from search pages like google.com. You pay to get your ad listed on their sponsored listings (in google's case, other engines are purely pay for placement). You set a bid price and it ranks you against other advertisers based on your bid. The idea is only targetted visitors will be clicking allowing you to get a higher conversion rate based on the traffic going to your website.