A back link is an inward link to your site that may or may not be reciprocated with an outward link back. If page A links to page B then a back link would be a link which goes from page B back to page A.
Backlinks are like a bridge that gives incoming way to a web page or site. A Backlink is a link that comes from another website to yours. The number and quality of Backlinks to your site can affect efforts to SEO Process.
There are many ways to getting back links like banner links, forum postings, comments on blog, submissions in article, classifieds just like that.