While explaining why computer processors have differences (specifically, why the Playstation 3 might be harder to develop for than the XBox 360 or the Wii), I touched on the idea of specialization and optimization.
Processors can be optimized for specific tasks, or they can be generic. The processor in your computer, for instance, can decode an MP3, or run a word processor, or display a webpage. The processor in your graphics card, however, can't do any of those things because it's heavily optimized just for graphics manipulation. It's incredibly good at graphics stuff. It might be able to decode an MP3, but that's not what it's designed for.
Just like how I know English, which is a very flexible, generic language, but a doctor has a very specialized vocabulary for his profession. He can say that someone has a fractured tibia, and that takes him two words. I could say the same thing, but I would have to say "His bone is sticking out of his skin!", with lots of hand gestures and some vomiting. I'm not optimized for medical stuff.