Almost every search engine use inbound links to rank a web page. Link popularity is the concept that most effects the search engine rankings. If a good site links to your page this tells search engine that there is some thing interesting and useful information on your page and it may give some popularity to your site. Inbound links also tells the search engine that your site is reliable and trustworthy. This trustability of your page depends on the page rank (different from search engine rank) that every search maintains. This page rank improves only with inbound links to your page. As many inbound links you get to your page the more the page rank will improve to your page. But all these inbound links should come from good and relative sites which already have some good page rank. The inbound links to your page must improve steadily but not rapidly. If you start building links rapidly search engines may think your page as spam and put your page in sand box.