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" />
A screen reader for the blind would be able to see read this image out as simply "An Image of my logo" and proceed on, otherwise the screen reader may have skipped the image completely or incorrectly identified the image as something unrelated.