check this link you cant get perfect idea for Anchor tags http://www.bhavanidirectory.com/Seo-Services/anchor-alt-tag.html
Anchor text is a text which describe your internal or the link you are placing. Like site interlinking
The anchor text is the text that overlies your link - either in backlinks to your website or on internal links within your website. In this link you see "the importance of mixing anchor text" - whereas the underlying link takes you to a different webpage: the importance of mixing anchor text In SEO terms, anchor text is vital in backlinks for telling search engines what the subject of a website/webpage is about and helping you to rank well for a term. The above link shows how important it is to mix up the anchor text you use on external links and shows how a website of mine had 2 months' work of link building wiped out for not mixing hte anchor text enough.
an anchor text amy also be called a keyword... virtually there is no limit of how many anchor texts you can put in a page. hope this helped you.
anchor text is the test use in trading link or doing link building to your website. Commonly we used our targeted keywords for our anchor tag.