The ALT tag is used in IMG SRC to provide a means of telling people what the image is about - like this:-
< img src="myimage.jpg" width="x" height="x" alt="the alt tag is used as a means of access for people who use text only browsers or for people who cannot see. It can be used to tell them what the image is displaying and where it links to if it's an image link. It is also a required tag under W3 recommendations, and disability law in both the UK and USA." >
Hope that helps:)