Danny Sullivan | Entrepreneur
This is an easy question to answer: Don’t buy links. That is, don’t buy links if you care about ranking well in Google in the long-term. They can seriously mess you up.
In very simple terms, links are like votes to Google. More links can help a site “win” the Google election for a particular search. Problem is, people mistakenly assume they can buy the election by buying votes. But the system is much more complicated than that.
Google allows some links to carry more weight than others, if they come from trusted sources. It also considers the words in likely near the links, to understand what a page being linked to might be about. Lots of people link to Amazon with the word “books” in the link, so Google effectively considers that a vote for Amazon to rank well for the word “books.” Since few link to Amazon using the word “cars,” you don’t see it ranking for that.
Google also discounts some links from sites it doesn’t trust. They might carry no weight at all. Worse, in some cases, it might decide that a link should be used to penalize a site, if it believes the site knowingly bought a link to try and “rig the election.”
Google can tell links are purchased in a variety of ways. For one, you know those out-of-the-blue emails that promise to sell you links that Google can’t detect? They get sent to people at Google, as well. Google also has an entire team devoted to doing nothing but investigating web spam, which includes people buying links.