No adverts on it? Have you ever used it? Go to it now and type in, say, "House" and look at all those advertisements on the right-hand side. Search again for something like "CDs" and you get the second type of advert - "Sponsored Links" at the top of your search results.
By only showing you adverts that have something to do with your search request (targeted ads), Google can charge a lot more for them. Much more than ordinary and untargeted banner ads, pop-ups and pop-unders.
It's still a cracking search engine, though. And if they want to make some money by putting adverts on, I don't really mind AS LONG AS:
i) It's clear what is and what is not an advert;
ii) Banner ads aren't used;
iii) Pop-ups and pop-unders are abolished and made punishable by death.