Google really makes a lot of money out of absolutely nothing they own. That is total genius.
Other players like yahoo have tons of their own content but fail to compete with google who
own absolutely nothing apart from a huge collection of hyper links.
  Google makes 2/3rds of revenue from search results. You know whenever you do a google search
you can see a result on the top that is highlighted and probably quite relevant as well. If somebody clicks on that link the money goes to google, the revenue varies from a fraction of a cent to $30 per click.
 Second is as you mentioned Adsense. To use adsense all you need to do is register your website to
google and google gives you few lines of code which you can paste in your web pages where you want google ads to appear. If someone clicks on the ads on your website google will give a fraction of revenue to you and keep the rest, hence they earn money without absolutely doing anything. Well it benefits the content owener as well, as reportedly some bloggers have earned millions of $$ this way.