What is meditation? Who is it done by? What is achieved by it? What is phys/mental going on inside their head?

Meditation can be done by anyone. Although traditionally its qutie often done by the religious there is nothing intrinsically religious about it. It involves sitting still for long periods of time , excluding outside things from your thoughts and concentrating on one thing only. Quite difficult as teh mind tends to wander. Its supposed to be good for stress but takes some practice. It also helps if you can comfortably sit in some yoga poses whilst doing it.