-Link building or backlinks :
are that links toward your site or your page
because its improve your page rank so improve your page or site and appear in the 1st pages of google for specific keywords
-What types of backlinks?
1-Dofollow ==>that which valuable role in improving your page rank
2-Nofollow==> which haven't role in improving your page rank ( in some cases its good)
-How i get backlinks?
social media, bookmarking sites , web 2.0, blog comments , forums , RSS feeds, directories submission and the best one is Article Directories