backlink = link to your site.

Backlinks are probably the most important factor for rankings. Essentially, when
a web page has a text or image that hyperlinks to your site - that is called a backlink.

A brief guide can be found at www.google.com/webmasters

Basically, you want to establish as many backlinks to your site as possible. You can buy, trade, or gain links naturally. There is much written on the subject.

Hope this helped.

Avi Wilensky

Well other sites linking back to your sites is back links in very simple terms, seo book and avi explained it very well. The aim is to have back links from sites with high page rank and sites which are somewhat related to the content of your site. The advantage of having good back links is that you do not have to pay or wait for a long time to have your site listen in SERPS. Since google bot will normally be visiting already established sites and follows links from there, on finding your link your site will be visited and thus indexed.

Backlinks is a links or sites linking to your site...

I think you have to be careful, were you link to. Random linking won't help your site any. From what I understand, the link should be from a site with related content or value. There are various ways to trade links, but I dont think it is always advantageous. We have a link on our website that allow people to copy and paste the code to link back to us, but we get quite a bit of spam. A lot of people asking to trade links, and place our link on some ramdom directory, not even on their site!
So be careful, and check and make sure the people that you do trade with actually place your link on their site.

Backlink is good to your site when it is came from relevant site or good PR site. But link came from nowhere doesn't help your site. It's a link farm.

A link from an unrelated site doesn't make your site or their site a link farm. It just makes the link less valuable from an SEO perspective.

backlinks are basically votes to your site.

Some search engines rank your site based on the amount of backlinks pointing to yours

Not the amount of backlinks, the quality of backlinks. Not all links are created equal.