AdMob is a mobile advertising marketplace that connects advertisers with mobile publishers. They allow advertisers to create and target ads with plenty of detail.
Ads can be targeted to locations, carriers, phone platforms and phone manufacturers. Ads can also be targeted to specific sites or you can browse their channel categories including categories like communities, contextual search, entertainment, etc. All ads are run on an auction-based pricing system. AdMob clients include ESPN, CBS, Geico and Starbucks.
AdMob was acquired by Google in 2009. Since the acquisition, Google has refocused AdMob's offerings, pointing advertisers to AdSense for mobile web advertising and to AdMob for the mobile app space.