The Google PageRank Concept
Google, the worlds most successful search engine, has been on the cutting edge of ranking websites since it's inception. When Google was invented, it was built on the concept of "PageRank", a proprietary system which factored in the "Popularity" of a page in addition to the many other factors which had already been used for ranking web pages by other search engines at the time.
The PageRank system uses advanced analysis of hyperlinks to determine the relevance and popularity of a given page for it's associated (or "target") keywords. The reason for which links are analyzed is quite simple: they are an "off-site" element which are significantly more difficult for a webmaster to manipulate. Before the concept of link analysis, only "on-site" material was used to score websites for their target keywords. For example, the "Meta Keywords" and "Meta Description" tags were used to determine what the primary keywords of a page should be, which meant that there was a lot of "spamming" going on in order to obtain better rankings. Today, such tags are close to useless.
Imagine Site A, a website specializing in computers. If some other website creates a link to Site A, this is considered a "Vote" for Site A. Since these links are usually given by 3rd party webmasters, they are significantly more difficult to obtain and manipulate, thus they are given more importance in the "Algorithm" (a computer mathematical formula), which determines a sites rankings.
The PageRank model which is known to the public is scaled from 0-10. 0 Represents an extremely low page popularity, where 10 represents an extremely high popularity. This PageRank level is calculated on a per-page basis, not on a per-site basis. This means that your homepage might have a PageRank 5, but some of the "internal" pages of your website might only have a PageRank 3, or perhaps even 0.
The more PageRank a given page has, the more it may pass on to other pages. If a web page has a PageRank 6 and has 6 links on it, each of the pages to which it links will receive an even share of that PageRank. But the PageRank system works in a "non-linear" model and so each of these pages would not have a PageRank of 1, but perhaps something more like 5. For the mathematicians out there, it has been hypothesized that the PageRank model is operating with a base power or 6 or 7, although these are just guesses by those studying this field. Only Google know for sure!
As time has progressed, the "PageRank Algorithm" appears to have become more complex. It now factors in several other components of links, such as the "ALT Text", link placement and other interesting variables, each of which are covered elsewhere in our articles.