Anchor text is the clickable text in a hyperlink. SEO best practices dictate that anchor text be relevant to the page you're linking to, rather than generic text. The blue, underlined anchor text is the most common as it is the web standard, although it is possible to change the color and underlining through html code. The keywords in anchor text are one of the many signals search engines use to determine the topic of a web page.
The anchor text is also known as the link label or link title. The words contained in the anchor text help determine the ranking that the page will receive by search engines such as Google or Yahoo and Bing. Links without anchor text commonly happen on the web and are called naked URLs, or URL anchor texts. Different browsers will display anchor text differently, and proper use of anchor text can help the page linked to rank for those keywords in search engines.