Backlinks are the incoming links to a website or web page and the number of backlinks is an indication of the popularity or importance of the website or a web page.
Search engines often use the number of backlinks that a website has as one of the factors for determining the website’s search engine ranking. For example, Google’s PageRank algorithm uses backlinks to help determine a site’s rank.
In today’s SEO world, more than 95% of the focus is on building quality backlinks.
BackLinkWatch lets you discover the number of sites linking to your site or web page. 
It also lists the page rank of the page linking to you. You can also find out about the links using the “nofollow” flag.
Also it seems to be using Yahoo Web Service to gather and list the backlink data.
The service is free to use and it allows you to check the backlink of any website. Go ahead and check your sites backlinks at BackLinkWatch.
