"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..."
In a sense, then, PageRank is like a giant electronic voting system. The page that gets the most votes gets awarded the highest PageRank (on a scale of 0–10). So, grossly oversimplifying, simple importance is deter-mined by link quantity.
This is not the whole story, however. Google goes on to explain:
Google looks at considerably more than the sheer volume of votes, or links a page receives; for example, it also analyzes the page that casts the vote. Votes cast by pages that are them-selves “important” weigh more heavily and help to make other pages “important.” Using these and other factors, Google pro-vides its views on pages’ relative importance.
To continue the voting analogy, Google is not a first-past-the-post sys-tem. Every vote is not equal. If you get a vote from another site that has already garnered many votes of its own, this will carry a greater weight-ing than a vote from a relative unknown. So, to complete the picture, rel-ative importance (or PageRank) is determined by both link quantity and link source importance.
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