I recently joined sponsoredreviews.com. When an advertiser has a product or service to promote online, he might want to generate blogger buzz about it spread the word. He will then pay a blogger to write a post about his product or service with links to his website.
This actually gives two party advantage. Advertisers can promote their product or service, while blogger can earn money, and have something to write about.
Advertisers and publishers could be penalized for participating in sponsored reviews. It can bring down your PageRank. One way to tackle this is by using the “NoFollow” tag within your links so Google doesn’t crawls the link.