What is RankReport?

RankReport queries 21 active Google datacenters for PageRank values for a given URL. Since the datacenters do not update in any predictable way, the output may reveal upcoming PageRank changes, sometimes days before they appear in all datacenters. This information can be useful for search engine optimization purposes.
Note: RankReport queries servers at different C-blocks. However, all servers may not be on different physical locations.

What is PageRank?

PageRank is a system for ranking web pages, developed by Google's founders Larry Page and Sergey Brin at Stanford University.
Google describes PageRank as:
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."
The algorithm used to calculate PageRank is called the Power method.