What is a backlink?

Backlink is basically incoming(inbound) links to your site.

Simple syntax of a backlink HTML code:

<a href="www.bestcarshop.com">Best Car Shop</a>

For example you have a website, www.bestcarshop.com and there are 2 websites that give links to your site in their webpages. In this case your site have 2 backlinks. 

Backlinks are one of the most important factors for SEO(Search Engine Optimization). The number of backlinks indicates the popularity or importance of a website or a page. This is used by Google pagerank(basically google pagerank is a score -that shows the quality of website or webpage- for websites and webpages between 0 and 10. 10 is the best value.) which show algorithm for calculation of pagerank. For example facebook, twitter have lots of backlinks from many different websites in the world so they have a good pagerank value. 

But backlink quality is important for better pagerank and SEO. For example your website www.bestcarshop.com is getting backlinks from facebook.com and xyzqwe.com(a low ranked, less unknown website). The quality of backlink from facebook.com is better than from xyzqwe.com

There are several factors that determine the quality and value of a backlink. Backlinks from quality sites on a given topic are highly valuable. If the backlink site have content related with your keyword topic, the backlink is considered relevant and believed to have strong influence on the search engine rankings of the webpage granted the backlink. For example your site www.bestcarshop.com have 2 backlinks. One from www.car.com and other from www.rentanoffice.com. The backlink from www.car.com is better than from www.rentanoffice.com because of the content of your website. - See more at: http://www.freebacklinkbuilder.net/what-is-backlink.aspx#sthash.00hHqr4W.dpuf