In general, whats the role of the cams in a car?

Why does putting performance cams increase power?

 

Cam shafts actuate the valves that allow fuel-air mix into the cylinders and the exhaust (results of combustion) out. You can find good explanations of internal combustion engines at www.howstuffworks.com.

Cams have two fundamental characteristics, lift and duration. Lift (from the old pushrod based engines, guess I'm showing my age) is how much the vavles get opened (as the dentist said ... open wide!) and duration is how long they stay open.
More lift, more duration == more fuel-air mix == more power. However, this is subject to the laws of diminishing returns ... too much of either is a bad thing.

-- dave