How do you change the Alt tag. Like when you hover something long enough the Alt tag pops up but how would you make one

<img src="your image" alt="what you want here">


For Links:
<a href="your link" title="what you want here">Your Link</a>


Or maybe you mean something like this:
http://www.dynamicdrive.com/dynamicindex5/popinfo.htm
http://www.dynamicdrive.com/dynamicindex5/popinfo2.htm

There are more like that on www.dynamicdrive.com

Thank you so much!