It's a practice that's commonly seen on blogs, especially on those of bloggers in the Internet Marketing community. A blogger is given a product and then writes a review of it, often linking to the product website or the product's creator.
Is this a paid link ? Google seems to think so and has ‘warned' bloggers and reviewers that it sees what they're doing.
On Friday, Google issued a statement on it's Webmaster Central Blog which lays out best practices for bloggers who review free products they received from companies.
Google's guidelines on the subject are pretty straightforward.
Bloggers are advised to use the nofollow tag when linking back to the product or the product creator. The rationale for this is that these are unnatural links. Google considers that these links are not organic and would not exist if not for the gift of the product.
Google goes on to say that bloggers should clearly disclose the relationship with the company that provided the product. Essentially this means that if a company gives you a product in exchange for a review then you should say so.
Finally, Google reminds bloggers to create compelling and unique content. This means cut and paste copy as provided by the company that provided the product is off limits. Content should be original and provide value.
Often when Google issues guidelines like these it's a signal that manual penalties may be handed out for non compliance. So if you're a blogger who regularly reviews products or services then it's probably time to go back and revise some of your content and ensure that future content complies with these guidelines.