Queries bout programmable matter

What exactly is a cellular automaton ? Please simplify as much as possible. I don't like what Wiki says.
What exactly is Quantum decoherence and why does it pose problems for quantum computing ? What exactly is a qbit ( describe it please ) ?

I have asked these questions for further information. more simplified information than I can get from google. So someone who really knows what this stuff is, please help me to understand.

Yes and also, when you have a quantum well, why does it release photons from the top P surface (P-N-P) ?