PageRank is a proprietary algorithm developed by Google to determine the usefulness of an individual web page. Ranking scores from 0-9, where 0 being the lowest and 9 being the highest rank.
Characteristics / Implications
The algorithm of PageRank is constantly changing to better reflect the usefulness of each web page. There are approximately 200 factors used to determined the PageRank score of each web page.
You can find out the PageRank of any webpage through the use of Google Toolbar.
Before Google was created, Yahoo and other search engines index web pages manually -- i.e. if you have a website and if you wish your website to be listed in Yahoo, you will have to submit the web address (URL) to Yahoo, and Yahoo will send someone to take a look at your web page and manually abstract crucial information from your site and list them on their search engine database. In the later years Yahoo and many other major search engines implemented a paid advertisement scheme that lists sponsored listings higher in the search results.