Backlink, is a SEO term and it means  a link from other site pointing to your site.

so lets say you have site A – your site.

and you have a site B – other site and

Site B – happens to have a link on its page, which looks like this computer repair

and when you click on it it goes to site A.

this means Site A has a backlink.

Backlinks are different in nature. There are stronger ones and there are weaker ones.

You always want to have links of the sites with higher page rank. When you do those links will give you more power than simple links from sites that have low pagerank.

- Relevancy – keep in mind category and relevance of the page that links to you. If you do web design and you want to get links, it would be wise to get links from similar sites of technology, computers, internet or that kind of sphere. Relevancy is not so specific, but at some point it is and it should be if you think about it.

- what text surrounds a backlink is also important. if an articles is about birds and your link is to computer repair, then its sort of obvious it could be discounted as a non relevant link also.