Anytime you pay someone to write about your website and they add a link in the article to your website is considered to be a paid link. Someone who does write about a website can add the rel= no follow attribute to the link and make sure to disclose that it is a paid advertisement to avoid any confusion.
Matt Cutts from Google has a great post entitled How To Report Paid Links. in it he discusses how to report paid links some companies that do this sort of paid blog posting and a lot more. No doubt worth a read time and time again for any SEO.