Actually 'alt' is not a tag, it is an attribute.
The alt attribute is a additional bit of information you can add to links (<a>) and images (<img>) to provide additional information about the tag. Generally 'alt' attributes are used by certain pieces of handicapped software or by SEO crawlers to help identify what exactly the tag is doing. For instance if you have the follow
<img src="/images/my_logo.jpg" alt="Image of my logo" />