Search engines use bots( computer robots) to crawl a site. It is also known as spider or crawler. When a crawler find a new website,it is indexed in search engines directory. After getting listed in a search engine directory its value is determined on various factors.
Google calls this value as PageRank. What are the exact factors to determine PageRank is only known to Google search engineers. One of the main factor is backlink. Backlink is a link from other site to a site. Generally those types of backlinks are counted that came from a relevant and high quality site.
Not every pages of a web site get same PageRank. Those type of pages get more PageRank that are more valuable to users in according to Google.
Other search engines like Msn works in same manner. So a page with more quality backlinks get a higher pageRank and likely to come up in top positions in search results for its relevant keyword.
