PageRank is a Google algorithm that ranks the importance of web pages based upon links between pages.
A page’s PageRank is not the same as its ranking in Search Engine Results Pages (SERPs). While PageRank is a foundation for Google’s search engine results it is only one of many factors used to determine SERP rankings.
For the purposes of PageRank each link to a page is essentially a “vote” for that page. More links to a page results in a higher PageRank. In addition a link from a page with high PageRank counts as more “votes” than a link from a lower ranked site.
The algorithm was developed by Google founders, Larry Page (and named for him) and Sergei Brin in 1996 while they were grad students at Stanford and formed basis of first Google search engine.
The term PageRank is a trademark of Google but the patent is held by Standford and exclusively licensed to Google.
Toolbar PageRank, so named because it was originally reported from Google’s Toolbar browser plugin, assigns a numerical value from 0 (least important) to 10 (most important) to pages and is influenced by factors in addition to pure PageRank. A variety of tools are available to access a page’s Toolbar PageRank, however the reported value is generally out-of-date.