In the year 2000, Google was slowly overtaking the search engines of that time (Lycos, AskJeeves, Yahoo) by virtue of their Page Rank based algorithm (named after co-founder Larry Page). This innovative algorithm used the links from one page to another to evaluate the importance of a website. They could now grade website based on how many pages have voted for them and used a metrics called Page Rank to denote this value for every domain and URL. They released the PageRank toolbar to make it very easy for SEOs to check the PR of their websites and the PR toolbar because the default tool used by SEOs all over. Domains were sold based on PR and SEO services rendered to improve this number. Any marketing place that has anything to do with SEO started using this one metric to compare websites.
Almost 15 years after that, Google decided to shut down this toolbar. They still continue to use the internal metric till this day and will do so for the unforseeable future. However, they no longer share the metric with anyone outside Google.
PageRankTool.net initially used the API that would get the PR data for the toolbar and when this was shut off, we have startyed using Open Page Rank data. We hope to continue providing a free fast service for SEOs and webmasters everywhere to check the Page Rank of their websites.