Google uses a method known as PageRank to quantify the "importance" of a web page. PageRank is measured on a scale of 1 to 10 and is primarily determined by the PageRank of links pointing to a given page. An exhaustive but technical explanation can be found here. PageRank is but one of several factors Google uses to place a web page in the search engine results pages.
Enter a URL into the form below and click "Get PR" to find the page's PageRank.