Diffusion vs absorption

[howiebrou]

Can any in easy to understand terms explain the necessity and theory behind using various diffusion and absorption panels and how they should be placed in a standard rectangular room with the speakers on the short side?

I have seen rooms with bass traps in the corners behind the left and right speakers, a diffusion panel in the middle, absorption on the ceiling and side walls at the point of first reflection etc and then seen some with it all reversed.

Any good starting point one could recommend? I don't want to buy loads of absorption panels to find they are of no use! Professionals whom I would trust on this are thin on the ground over here.

Thanks

howie