I'll just add a little detail to Erik's response:

Advancing or retarding the cam moves the engine's torque band around the rpm scale by moving the valve events further ahead of or further behind the movement of the piston.

Advancing the cam clockwise in relation to the crankshaft improves low-end power and response.
Begins and ends all valve opening and closing events sooner in relation to piston position.
The engine builds torque quickly, then lays down on the top end. Makes the motor "peaky".
Decreases piston to intake valve clearance.
Increases piston to exhaust valve clearance.

Retarding the cam counter-clockwise in relation to the crankshaft improves high-end power.
Delays all valve opening and closing events in relation to piston position.
Increases piston to intake valve clearance.
Decreases piston to exhaust valve clearance.

As has been stated, cam grinders will grind in a little advance when they manufacture the camshaft. You can figure the amount of ground-in advance by subtracting the intake lobe centerline in degrees from the lobe separation angle in degrees. Here's an example. Look at the bottom of this cam card. It shows that the intake lobe centerline is at 105* after top dead center and that the exhaust lobe centerline is 115* before top dead center. To get the lobe separation angle of this cam, add those 2 figures together and divide by 2.....
105 plus 115 equals 220, divided by 2 equals 110*. So this cam is ground on a lobe separation angle of 110*. If we subtract the intake lobe centerline (105) from the lobe separation angle (110), we find that the manufacturer has ground in 5 degrees of advance.
http://www.cranecams.com/?show=brows...tType=camshaft

Pretty neat stuff, huh?