The OS and the computers itself is unified into one corporation. Also, you get a full UNIX command line on OS X 10.5 or up.
Macs are more for business, programming, or for the metallic, luxurious looks. When you buy a Mac, you get the rights to run OS X on that computer, and not on any other computer unless it's a Mac also.
The Mac Pro is a computer designed for resource-intensive computation, eco-friendliness, reliability, and more recently, compact size. (I'm not sure how much CPU/GPU load the cooling system can support, though.)
The Mac Mini is just a powerful nettop (compared to some of the other nettops out there) that's energy-efficient. Take it on-the-go along with a keyboard, mouse, and a small monitor, use it as a TV media center, or use it as an easy-to-set-up home server.
Don't use them for a game server or public service unless it's running Linux or Windows.