Back-links are the cheapest method to get traffic to your web site.
A primitive example - if you were a blogger you might wish to go to another blogs, which deal with the similar topic as your own blog.
Going there you might wish to post a comment regarding your favorite topic, hobby and so on. Curious or furious owners go back to your blog and generate traffic to you.
Visitors of both sides generate traffic for them in this win-win situation.

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