Flat Rate PPC
Flat Rate PPC is where the advertiser and publisher (search engine) agree upon a fixed amount that will be paid for each click. The publisher has a rate that determines the cost per click or CPC based on the competition of the term, meaning how many other people want to pay for that click too.
With Bid-based PPC, the advertiser signs a contract that allows them to compete against other advertisers in an advertising network such as Google AdWords or Microsoft AdCenter. Each advertiser sets the maximum amount that he or she is willing to pay in an auction for a given ad spot based on a keyword . The auction proceeds automatically by a visitor clicking on the ad spot that the search engine results page (SERP) displays.