In the crawling (also called spidering or robotics) process, the search engine begins to discover the web or the pages of the overall sites on the web. It performs this by beginning to download pages from some sites. Regularly, it will begin with some small sites stored in its database. When it crawls some initial site, it will observe the links in those sites. If the search engine discovered new link that is not stored in the initial database, it will append it to the list to be crawled later. In the new links discovered, it may also discover new links that will be appended also to the list of the sites that will be crawled soon.
One may imagine that only the keywords on the page controls the rank but recently there are a key factor that controls hat ranking which is the backlink. Mainly the concept of backlinks is related to voting and reachability keywords because an existent link to a page mean that the page is good for that site and this it effectively votes for it. Also the page will be reachable from that site. Recently, the search engines are concerned with the concept of reachability, they say that if one browse randomly through the sites on the web, what is the probability of reaching to certain site. This probability mean a higher ranking if it is high and the reverse is true.Thus we find that the ranking depends on the backlinks also.