Here’s a little information about how search engines like Google index and rank web pages.  It’s taken from the ebook Optimize your WordPress Site by the SEO wizards at Yoast.com.

Links
Search engines like Google follow links. It follows links from one web page to another web page. A search engine like Google consists of a crawler, an index and an algorithm. A crawler follows the links on the web. It goes around the internet 24/7 and saves the HTML-version of a page in a gigantic database, called the index.

 

This index is updated if Google has come around your website and found a new or revised version of it. Depending on the traffic on your site and the amount of changes you make on your website, Google comes around more or less often. For Google to know of the existence of your website, there first has to be a link from another site to your site. Following that link will lead to the first crawler-session and the first save in the index.

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Google’s secret algorithm
After indexing your website, Google can show your website in the search results.
Google has a specific algorithm that decides which pages are shown in which order. How this algorithm works is a secret. Nobody knows exactly which factors decide the ordering of the search results. Moreover, factors and their importance change very often. Testing and experimenting gives us a relatively good feel for the important factors and the changes in these factors.