What are backlinks?

Backlinks are urls that direct visitors to your webpage (also called as inbound links).
Back links are the cheapest way to have traffic to your site.
A primitive example - if you were a blogger you might wish to go to other blogs, which deal with the same topic as your own blog.
Going there you might wish to post a comment about your favorite topic, hobby and so on. Curious or furious owners go back to your blog and generate traffic to you.
Viewers of both sides generate traffic for them in this win-win situation.