You are talking about 2 different things in as many sentences.
The "alt" tag, or alt text, is text that displays on the page instead of an
image, an "alternative".
This is widely supported by browsers, it is used to describe an image if the
user has image support off in their browser and for blind users.
A screentip or tooltip, is the "title" tag. The title tag displays text in a
yellow pop-up when the mouse cursor hovers over the tagged item. It is an IE
tag. You can refer to
http://www.devguru.com/features/knowledge_base/A100216.html