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How does Google classify a page?

The first search engine that started using a system of inbound links was Google, with an algorithm called PageRank.
PageRank is assigned individually to each page of a web site, determined by the other pages linked to this one.
There is a index of merit for every page on the Internet, calculated with an average of PR of other pages linked.
The result is the "importance", or "popularity", of a page.

How a PageRank is calculated

The algorithm Google uses is a secret one. Nevertheless, it is possible to understand how Google calculates this value.
To start, let's see how users navigate the net.
They start from a page, click and jump from a page to another one according to the links they find.
Let's say they start from any page; at a certain point they will click a link that takes them to another page.
There is however the chance that the user will stop to navigate and read a page without following the outbound links.
PageRank estimates it to be the 0.15 on each page. The possibility for a user to follow a link is then the 0.85.
If the user navigates for a long period of time, he will visit the pages that are most popular.
PageRank of a page is then the probability for a user to visit it.

Every page published on the net has a value above zero, even if very low. There is always a small possibility that someone writes the URL in their browser to visit the page.
Each page having a PR gives part of it to each page linked to it.

A filter is applied to every link to attenuate the PR of the 15%. This is like the probability of a user to leave the page without clicking on any link.
Links from pages with high PR have a higher PR, possibly if they are sites with similar subject.
It is very important not to have too many outbound links, expecially on the home page.
PR goes even from a page to another one in the same site.
It is very important to have many sites linking to yours.

How PageRank influences the indexing

After a search engine finds useful documents, they will be ordered using the PR, because they consider a page with many outbound links more useful.
The algorithm PageRank, consequently, pushes popular documents higher.

What is PR useful for?

It is useful to control the popularity of a site. It doesn't give exact information about the pages that link it, but an immediate result to understand how popular a site is.
To reach 4-5 is a good result, 6 represent a very good site, 7 is rarely reached by a normal webmaster.
PR 8-9-10 are reached only by big multinational companies.
If, looking for a certain keyword, you find sites with PR 7-8 on the first page, a site with PR 4 will find difficult to reach that position. Tags:Google PageRank PR indexing

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The ultimate search engine would basically understand everything in the world and it would always give you the right thing. And we're a long, long way from that.

Larry Page

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