First off a little disclosure this article over laps the Backlinks category of the FAQ.
What Are Backlinks?
Backlinks are links from other sites. Think of them as votes of affirmation. Only one vote can come from a domain so for SEO purposes it doesn’t matter if there are 100 or 1 link from the same domain one link is the count you gain. Subdomains are viewed separately…thats why yourblog.tumblr.com isn’t a tumblr backlink. Now those other links may increase traffic to your site but in regards to SEO value its one vote. Some call this metric