For the simple reason that Google was developed for school purposes, at Stanford University, enough information on how it works was published in papers. Therefore, a lot can be determined on how it works. Meanwhile other search-engines, which were developed primarily for commercial purposes, tend to keep their techniques publications at a minimum. From what is published by Google, a clear understanding can be generated about how Google itself works and also gives a general idea on how other search-engines may work.
What made Google successful was the drastic improvement in their algorithm to make sure to present the user with the highest quality pages in relevance with the search query within the first top 10 results. This has mainly been emphasized since it had been noted that users tend to look only through the first result page, usually having ten result links.
Google uses a lot of different factors to determine which pages are of better quality than others. PageRank is one of the most affective factors used by Google’s search-engine. PageRank is assigned to every page Google crawls (visits and indexes) and can be affected in various ways. The PageRank of a page can be determined on the amount of other pages linking to it, by who links to it, how many hits it receives and the page relevance data. Each page containing links is able to pass on its PageRank to the outgoing links so a link from a very high PageRanked page may be considered to be of better importance than hundred links from low PageRanked pages.
Please note that all information has been provided by: About SEO Malta which is an open source SEO information website.