Anchor Text Backlinks
Anchor text is the visible, clickable text contained within a link. Google, and other search engines, view anchor text as a key indicator as to what the linked site's content is about. Therefore, Google and others highly value the anchor text in backlinks when ranking sites in their algorithms. Typically, the more often a keyword appears in the anchor text of backlinks, the more likely it will rank well for that keyword phrase in Google.
An illustration of how powerful inbound links can be is found in an old phenomenon called "Googlebombing", where anchor text backlinks were used to improve rankings for websites often unrelated to the search phrase.