A cell processor is a cluster of processors. Have you heard of dual cpu computers, and servers using multiple processors? Unisys makes servers that can use up to 64 processors (hot swappable, too).
A cell processor is essentially combining the technology of multiple CPUs into one cohesive CPU (which is really more like simply a cluster of CPUs). It can be very powerful, as you can assign different procedures to different processors. For instance, you could have one bad-guy's AI on one CPU, another bad guy on another CPU, and when those bad guys aren't present simply assign another AI to one. I'm sure you can think of other examples where using an entirely separate processor for different processes can be useful, as you basically avoid cpu strain.
The problem, of course, is that developing for such a system is a royal pain. Only recently have more companies been including dual cpu support for very mainstream applications. Programming is even more of a pain as you have to tell the system where you want the code to run, and make sure things aren't conflicting. For instance, if you have a 1 room house, and need to tell 4 friends where to move 5 pieces of furniture, it's easy -- 1 room, 4 walls. But imagine if you had 4 friends, 5 pieces of furniture, and 20 different rooms. It gets a lot more complex making sure everything ends up in the right spot, not to mention avoiding collisions.