Google makes up to 90 percent of it's revenue through advertising with 68 percent of that revenue coming from advertising in there search results while the other 22 percent of their revenue comes from their adwords program where publishers sign up for an Adsense account and then place advertisements from companies on there website and earn money from clicks on those ads.
Google then takes a cut of that money per click and then shares the rest with the publisher.
The rest of there Revenue comes from sales of other products and services.
It's estimated that Google rakes in $15.5 billion dollars per year in Advertising sales alone.