Although I have no actual experience with it, I've heard it described as a noticeable sort of notch like bump. To stimulate it you massage it kinda like you would the g-spot or something.
Just googled for "prostate stimulation" and found this:
http://www.prostate-stimulation.com/
I once read a really great article on nerve.com about Aneros, an anal toy, but it is no longer available. Here is the link for it if you felt like subscribing:
http://www.nerve.com/regulars/ididitforscience/aneros/
<hr>Apologies for not being concise. Aneros is available. The article about it is not.