One of the "black-hat" Search Engine Optimization (SEO) techniques of recent years has been to employ automated scripts or real people to post comments that contained a link to your website on blogs. The start of this trend coincided with the rise in popularity of blogging, and it wasn't relevant to such marketers whether blogs were relevant to their content area or indeed if the comments were relevant to the page they were commenting on. The important thing (to these marketers) was that they were adding back-links to websites with relative ease, with the hope that their own search engine rankings would increase as a result.
Once search engines became aware of this practice, there was fear among the respectable commenting and blogging community that any comments containing links would associate their website with spam. But in this interview, Google's Matt Cutts (head of the Webspam team) says that, as with most SEO techniques, you shouldn't worry as long as you are not trying to abuse the system.