One thing that is very common for bloggers to do is to review a product or a service. As a result, one method of advertising that companies employ is to actually pay popular bloggers to review their product or service. And to make sure there is no conflict of interest, the blogger generally discloses the fact that they were paid to write the review.

Often times, bloggers will include a disclosure statement in situations that are less obvious that direct payment for a review. For example, on my blog (Daily Beer) I review a different beer every day. If a brewery decided to advertise on my blog, and it was a brewery whose beer I had reviewed, I would add a disclosure statement to that post that said something like "XYZ Brewery is a sponsor of Daily Beer. Their sponsorship of the site in no way impacted the contents of this posting."