The alt attribute (yes, it’s an attribute, not a tag) contains an alternative description, and is required for images and image maps. It can only be used for the img, area, and input elements (and the deprecated applet element). For the input element, the alt attribute is meant to be used for graphical submit buttons: <input type="image" src="image.gif" alt="Submit" />.
Use the alt attribute to provide text for visitors who, for whatever reason, can’t see the images in your document. This includes visitors using browsers that cannot display images or have image display disabled, visually impaired visitors, and screen screen reader users. Alt text is to be used instead of an image, not as additional information.