The next two paragraphs come directly from Google:

PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page's value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves "important" weigh more heavily and help to make other pages "important."

Important, high-quality sites receive a higher PageRank, which Google remembers each time it conducts a search. Of course, important pages mean nothing to you if they don't match your query. So, Google combines PageRank with sophisticated text-matching techniques to find pages that are both important and relevant to your search. Google goes far beyond the number of times a term appears on a page and examines all aspects of the page's content (and the content of the pages linking to it) to determine if it's a good match for your query.


According to Google, a page may be assigned a PageRank of zero for a variety of reasons, such as:

1. Google's spider hasn't yet crawled this site, or the site has only been developed recently.
2. There are no or few trusted links to the site.
3. Google believes the site violates their Webmaster Quality Guidelines.

When your site is ready:

1. Have other relevant sites link to yours.
2. Submit it to Google.
3. Submit a Sitemap as part of our Google webmaster tools. Google Sitemaps uses your sitemap to learn about the structure of your site and to increase our coverage of your webpages.
4. Make sure all the sites that should know about your pages are aware your site is online.
5. Submit your site to relevant directories such as the Open Directory Project and Yahoo!, as well as to other industry-specific expert sites.

Design and content guidelines:

1. Make a site with a clear hierarchy and text links. Every page should be reachable from at least one static text link.
2. Offer a site map to your users with links that point to the important parts of your site. If the site map is larger than 100 or so links, you may want to break the site map into separate pages.
3. Create a useful, information-rich site, and write pages that clearly and accurately describe your content.
4. Think about the words users would type to find your pages, and make sure that your site actually includes those words.
5. Try to use text instead of images to display important names, content, or links. The Google crawler doesn't recognize text contained in images.
6. Make sure that your TITLE and ALT tags are descriptive and accurate.
7. Check for broken links and incorrect HTML.
8. If you decide to use dynamic pages (i.e., the URL contains a "?" character), be aware that not every search engine spider crawls dynamic pages as well as static pages. It helps to keep the parameters short and the number of them few.
9. Keep the links on a given page to a reasonable number (fewer than 100).
   
   

For more information about Google's Webmaster Quality Guidelines, please visit Google's Webmaster Tools pages