If you are in the process of marketing your website to search engines, a process known as SEM, Search Engine Marketing, then you have probably already heard about Link Enhancement, or Link Building.
 
What you probably haven’t heard, is that it MIGHT be illegal. well, not illegal according to the law of your country, but illegal in the eyes of Google. Specifically, sites that charge to post links and sites that purchase links. This is what Google has to say about the practice of Link Building:

We work hard to return the most relevant results for every search we conduct. To that end, we encourage site managers to make their content straightforward and easily understood by users and search engines alike. Unfortunately, not all websites have users’ best interests at heart. Some site owners attempt to “buy PageRank™” in the form of paid links to their sites. Buying links to improve PageRank violates our quality guidelines.

Google uses a number of methods to detect paid links, including algorithmic techniques. We also welcome information from our users. If you know of a site that buys or sells links, please tell us by filling out the fields below. We’ll investigate your submissions, and we’ll use your data to improve our algorithmic detection of paid links.

 What Google is talking about in this policy refers to websites that charge a fee to list your site, such as these websites. Google is also referring to the websites that pay to be listed on those sites.
 
Google however is NOT referring to the practice of link building or link enhancement, which is to essentially go around the internet to forums, discussions groups, blog comments sections etc.. and post a link to your website.
 
That practice is ok. So is the practice of exchanging links with other websites. So, is it acceptable to pay a company to build links for your website? YES! Is it ok to pay a company for posting a link to your website? NO!
 
There is a difference.