Here is the general page rank algorithm:

But of course there are simpler ways of explaining this:

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, search engines interprets a link from page A to page B as a vote, by page A, for page B. But search engines looks at more than the sheer volume of votes, or links a page receives, they also analyse the page that casts the vote. Votes cast by pages that are themselves "important" weigh more heavily and help to make other pages "important".