rear diffuser
 

I was wondering if anyone can explain the purpose of these(rear diffusers)...thanks in advance.

Downforce, without the penalty of drag. (or without nearly as much drag as an equivalent wing) They are simply more efficient.

try www.mulsannescorner.com for some aero info...

M.

Arent they also supposed to stabilize the air comming out from under your car so that it results in your car being more stable also?

Thats what I heard - don't quote me on it.

no.
they utilise the ground effect (on road cars, very minimally as the diffuser is often too far away from the ground) to basically increase the amount of downforce produced. that's what makes your car stable. no, the flows behind your car after it passes through a body of air are definitely not more stable.