Comparison Shopping Engines
Product engines (a.k.a. product comparison engines or price comparison engines) are search engines for products, and let advertisers provide "feeds" of their product databases. When a user searches for a product, links to advertisers are displayed for that particular product. More prominence is given to advertisers who pay more; however, the user can typically sort by price.
Some product engines such as Shopping.com use a pay per click model and have a defined rate card. Other engines such as Google Product Search, part of Google Base (previously known as Froogle), do not charge for the listing, but still require an active product feed to function.
The following are notable PPC product engines:
Service engines
Service engines allow advertisers to provide feeds of their service databases. When a user searches for a service, links to advertisers are displayed for that particular service. More prominence is given to advertisers who pay more; however, the user can typically sort by price or other criteria. Some pay per click product engines have expanded into the service space, while other service engines operate in specific vertical markets.
The following are notable PPC service engines: