For search engine spiders or robots or google bot, links are like the highways between the pages. Since the late 1990s, search engine have treated links as vote of trust to your website.
Links are not everything in SEO but a play a major role in the website rankings. Through links, search engines analyze trust, spam, authority and popularity of the website. Links are a very good way of identifying expert documents on a given subject.
Directory submission is one of the best way to build high quality backlinks. In SEO, directory submission is the basic step taken by a search engine optimizer to improve the popularity of the link. Here is the list of top 166 high PR directory submission sites list.