I'm not sure why Microsoft decided to call the buttons hover buttons.
I prefer the term rollover image, or rollover button, as used by Macromedia's Dreamweaver.
They are good for making navigation bars for websites. When a user points to one of the buttons on the navigation bar it changes.
There are plenty of good tutorials on the internet on how to use hover buttons. Just do a search on google for "hover button tutorial".
Another good source of information is the Microsoft website.