A tube pan (as you might use for angel food cake) with fluted sides. You may see copper bundt pans used for decoration, or bundt cakes with fancy toppings used to show the refinement and class of fancy cooking spreads in magazines.
I do not believe that it is used for any particular type of cake. A quick Google search will turn up more bundt cakes than you can shake a spatula at, but all that they seem to have in common is that they are cakes made in a bundt pan. They tend to be heavy cakes, and have all kinds of fruits and spices mixed in.
www.bartleby.com says that 'Bundt cake' was originally a trademark.'Bundt' is too new a word to show up in my home dictionary (1966). I have an off-line source that informs me that bundt should be pronounced 'Boont'.