Bypass capacitors, 2 voltages, 1 ground
The capacitor doesn't have to be literally on the pins, but should be (as everyone has indicated) as close to the IC as possible.

As for the 3.3V vs 5V: You want to use a capacitor that has a voltage rating above that which it will routinely operate at, to handle the occasional spike. Most small through-hole ceramic capacitors will be rated at 25, 50, 100V or more, so you should have no problems using a given bypass capacitor on either your 5V or 3.3V supply. (Be aware that surface mount ceramic capacitors voltage ratings can be much lower, 6.3, 10, etc.)

For more information about bypass capacitors, there's a good article at Seattle Robotics.