Let's say you created glossary of terms related to your business something genuinely useful to other people in your industry. A few bloggers discover the glossary and think it's worth talking about so they mention it in their blogs and the link to it. Thats a real links. This kind of link building encourages people to link to your site and is often known by the term "link bait". On the other hand let's say you buy links.
When you buy links, you pay other site owners to link to your site. Those are the artificial kind of links. The site owners don't really care about your site. They don't care what the link is pointing to. They're simply placing the link in exchange for money. Of course it's often difficult for the search engines to determine the difference between a real link and an artificial link.