"Search!" is the answer commonly spit out, it's the obvious one and the one that the standard narrative wants you to believe: Google is the leader in internet search, so that's gotta be it, right? But that's not it, hence the second question that comes up in conversation: How does Google make money?
The answer to that one is the answer to the first one as well: Google is an advertising company. Yes, it started in search, but search, maps, blogs, and everything else it does (and it does a lot) is secondary to ads, which bring in over $10 billion a year. For most of the Internet, Google is advertising.
So then comes the third question: Then why do they do so many other things? Rough Type's Nicholas Carr posits a very good answer:
Google’s protean appearance is not a reflection of its core business. Rather, it stems from the vast number of complements to its core business. Complements are, to put it simply, any products or services that tend be consumed together. Think hot dogs and mustard, or houses and mortgages. For Google, literally everything that happens on the Internet is a complement to its main business. The more things that people and companies do online, the more ads they see and the more money Google makes.