You take a set of bracketed shots of a scene in order to capture the wide tonal range, and this program combines them for you. It's sweet.
For example, it took these two photos and turned them into the lower one:
Here's an example of three images I took:
Here was my attempt to blend them:
And here's what Photomatix did:
I know it's not completely realistic, but it sure does look cool.
Here's another example.
My blending and Photomatix's blending both have their pros and cons, but Photomatix is about a million times faster and makes some pretty cool images, even if they are a bit soft and unrealistic.