If you do not know what a backlink is you have certainly heard another blogger or Search Engine Optimization (SEO) expert mention it. Here are two good definitions:
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 . Backlinks are also known as incoming links, inbound links, inlinks, and inward links. (Wikipedia)
The backlink is essentially the same thing as an inlink. It is a link on someone else’s site that leads to your own, and it’s extremely important to garner a large number of backlinks that are reputably obtained for several reasons. First, in SEO or search engine optimization, part of the algorithm for ranking your page on search engines is determined by the number and quality of backlinks to your site, providing they occur naturally and you have not purchased them. Second, backlinks are how you get known on the Internet. If lots of other sites provide a link to yours, a natural increase in traffic to your web pages occurs. (Wisegeek.com)
Here is a post I read on backlinks today. If you have other references you would like to add please post a comment.
Can too many backlink have negative effects on your site?
I think the wrong types can
The negative effect of backlink is that you are posting and spamming without knowing that your site was being ban. Also if you post a good quality backlink that will be good.