How the Value of a Link May Differ Based upon Link and Document Features and User Data

"One of the signals Google uses to rank web pages looks at the links to and from those pages, to see which pages are linked to by others. Links from “important” pages carry more weight than links from less important pages. An important page under this system is one that is linked to by other important pages, or by a large number of less important pages, or a combination of the two. This signal is known as PageRank, and it is only one of a large number of ranking signals that are used by Google to rank web pages and determine how highly those pages show up in search results in response to a query from a searcher."

Google's Reasonable Surfer: How the Value of a Link May Differ Based upon Link and Document Features and User Data