ALT tag is a attribute..it is show when you put this tag on image when you move your cursor on image then you see this alt tag on that particular image...
The ALT attribute provides alternative information for an image if a user for some reason cannot view it. Now days web browser's are available in a very wide variety of platforms with very different capacities. some of your visitors cannot see image,be they blind,low sighted, the ALT attribute is of great nup for those people that can rely on it have a good idea of what's on your page.
Alt attribute is used to give information to search engine about your image i.e what actually image is For eg:-<img src="images/logo.png" border="0" alt="Logo" /> Main benefit is that you search engine only crawl text and due to alt your image is crawl