First, you get paid for reviews. Some sites deny it, but they get paid. If not directly, then indirectly through very high priced advertisements they used to pay less for. Then, if you have a big site that makes an impact on the market, companies like to give nice presents. Many big site owners get their "commission" this way.
Another way is bby selling products like newegg or selling items for say amazon.com from a person's site, I think they offered 5% a few years ago when my site was running different than now. The other way is ads, but you have to be careful who you pick. Google ads are a good choice and reliable. Some other companies are horrible when it comes to ads, pricing, etc. Basically, you can't live off of ads on a website for the MOST part. Large sites sell their own ads in the thousands of page views, this is the most efficient compared to all the small to medium sized websites that go through a company in the middle who takes most of the profit.
Do you have any specific questions? If so, I'd be glad to help you out, I spent a couple years running a fairly large website and learned lots of these things, feel free to PM me too.