How do we get paid for blogging?

There are lots of web sites which can pay you for blogging. For this you have to write sponsored reviews about the topics as told by them (like one I am writing :) ) in your blog. To do this you must have a nicely maintained blog. Good pagerank and traffic can increase the amount you can earn out of it. Sponsored post is different way for advertise on blogs. How this system actually works? How they managed to pay us $5 to $20 per post on an average?

All the sites under this category have one advertiser's section, which is the actual source of income for them and bloggers. They actually earns commission from what they get from advertisers. Now if we see all three sides of this business then:

Advertiser: Gets lost of reviews written for his product which makes his site more popular and optimized for search engines which drive lots of traffic to his site. The more traffic he have the more new customers he will get.

Bloggers: Get paid for writing sponsors reviews. Usually its a good amount and worth the effort put for making the sponsored post.

Paying web site: Earn money from the commission out of the amount paid to blogger by advertiser (Advertisers don't pay directly for bloggers but pay on basis on number of posted written for them).

I recently started posting sponsored reviews on my blog. Will keep on posting about my experience. So far I didn't make any money.