The alt attribute is used to define an "alternate text" for an image. The value of the alt attribute is an author-defined text:
<img src="boat.gif" alt="Big Boat">
The "alt" attribute tells the reader what he or she is missing on a page if the browser can't load images. The browser will then display the alternate text instead of the image. It is a good practice to include the "alt" attribute for each image on a page, to improve the display and usefulness of your document for people who have text-only browsers.