Page rank is a very important aspect of SEO. Search engines rank pages based on their relevance, uniqueness, and importance. A number of factors go into determining this including:
- The content contained within each page and across the website
- The structure of the content and code
- The use of internal links
- The number and relevance of external links or backlinks
- The amount of time a site has been registered
- How often content changes
When all this information is collected and analyzed each page is assigned a rank (for google this is between 0-10 with 10 being the best). Generally the higher the rank the closer to the top of the search results list your page will appear.
This sounds easy however there are penalties if these techniques are not executed within certain guidelines. For example, if the content is too repetitive, not in proper sentence form, or formatted in a deceptive manner the search engines will penalize you and decrease your page ranking.