PageRank is an algorithm used by Google Search to rank websites in their search engine results. PageRank was named after Larry Page, one of the founders of Google. PageRank is a way of measuring the importance of website pages. It is is one of the methods Google uses to determine a page’s relevance or importance. Important pages receive a higher PageRank and are more likely to appear at the top of the search results. Google PageRank (PR) is a measure from 0 – 10. Google Pagerank is based on backlinks.
Wiki has a nice technical report Wiki PageRank.