Actually, the charger which comes with iPhone/iPod is rated 5W with an output of 5V at 1A (where 5 watts = 1 volts x 5 amps); the one which comes with iPad is 10W with 5V, 2.1A.
However, do not confuse the rated capacity of the charger with the actual power delivered to the device. The second charger can supply *up to* 10W to the device, but the device only draws the amount of current (amps) it needs.
Thu , the iPhone/iPod will still take a 1A charge at 5V and at their normal rate of recharging, if you use it with an iPad charger. Conversely, if you charge an iPad with the iPhone/iPod charger, it will charge slower (and with the screen turned off) because the charger is only driving half of the amps it needs to the device.