What is a backlink?

“In basic link terminology, a backlink is any link received by a web node (web page, directory, website, or top level domain) from another web node (Björneborn and Ingwersen, 2004). Backlinks are also known as incoming links, inbound links, inlinks, and inward links.” ...... wikipedia….. useful as indicator of ‘popularity’ of the ‘receiving’ web node…...

What @virtualist said, but a little simpler: ‘The more links Google finds on the web that lead to your site, the better Google ranks your page’.

@virtualist & @markyy are both spot on. Backlinks are definitely a major factor in determining popularity/usability, but they can be as damaging as they are helpful in getting your site ranked. Each backlink aimed at your site has a value, for instance a link from a highly trusted site like a university homepage would be wildly more valuable than a link from a friends WordPress blog. On the flip side of all these “helpful” links your site can also loose position on the search engines or be removed from search queries all together if you have links from websites deemed “spammy” or “untrustworthy.”

The moral of the story is this: be careful who you peruse for links and for that matter how you peruse them… you don’t want to be thought of as a spammer do you? Backlinks are definitely not a black and white issue, so do your research and you will be fine.

SEOmoz.org
SearchEngineLand.com
MattCutts.com

In simple words, backlinks are links leading to your site from other sites all over the internet. They promote your site on the internet and also help google to give you a good page rank. However, you must know how and where to plant the backlinks, or else they may work against you. Beside backlinks, there are other ways to promote your site. A SEO expert will tell you. I usually consult http://www.freelancewritingdesk.com for my SEO work. Although it takes some time to get your site on the first page, a good SEO expert can help.

To add to previous responses, you can build backlinks to your site every time you post an article, with your site URL in the resource box, to an article directory. The more articles you post, the more links back to your site. Try some of these article directories: http://www.GoArticles.com, http://www.ezineArticles.com, http://www.ideamarketers.com, http://www.searchwarp.com