April 11, 2011 - Google has always used a number of signals in determining relevant search results. Some of these are on the pages themselves (such as the text on a page), some are on other sites (such as anchor text in links to a page), and some are based on user behavior (for instance, Google gathers data about how long pages take to load by using toolbar data from users who access those pages).
In recent months, Google has launched two ways for searchers to block particular sites from their search results. The first was a Chrome extension. More recently, Google has launched a block link directly in the search results that appears once a searcher has clicked from the results to a site and then return to the search results.