In everything, Google makes their money from Ads. Google purchased a company called Admob for $750,000,000 that serves mobile ads. You may notice that many of the android apps are free and ad-supported. Those ads are usually served through the Admob service platform.
AdMob - Wikipedia, the free encyclopedia
From Google's most recent SEC filing:
How We Generate Revenue
Advertising revenues made up 97% of our revenues in 2008 and 2009, and 96% of our revenues in 2010. We derive most of our additional revenues from offering display advertising management services to advertisers, ad agencies, and publishers, as well as licensing our enterprise products, search solutions, and web search technology.
Google AdWords is our auction-based advertising program that enables advertisers to place text-based and display ads on our websites and our Google Network members’ websites. Display advertising comprises the videos, text, images, and other interactive ads that run across the web on computers and mobile devices, including smart phones and handheld computers such as netbooks and tablets. Most of our AdWords customers pay us on a cost-per-click basis, which means that an advertiser pays us only when a user clicks on one of its ads. We also offer AdWords on a cost-per-impression basis that enables advertisers to pay us based on the number of times their ads appear on our websites and our Google Network members’ websites as specified by the advertiser. For advertisers using our AdWords cost-per-click pricing, we recognize as revenue the fees charged to advertisers each time a user clicks on one of the ads that appears next to the search results or content on our websites or our Google Network members’ websites. For advertisers using our AdWords cost-per-impression pricing, we recognize as revenue the fees charged to advertisers each time their ads are displayed on our websites or our Google Network members’ websites. Our AdWords agreements are generally terminable at any time by our advertisers.