There are various chargers for the iPad, iPhone and iPod. They will ALL work, but chosing the wrong charger will result in extended re-charge times. The iPad should use a 10W power brick, the iPhone and iPod a 5W power brick. Unfortunately, some of the power bricks that look identical are 5W and some are 10W. You can read the wattage on the 10W power bricks right on the unit, but the information is light grey on white and hard to read. It will NOT damage any device by using the wrong wattage power recharger, but it will be slower if the recharger does not output the correct wattage( e.g. 10W for iPad, 5W for iPhone and iPod).
Remember that the USB port on a computer typically puts out 2.5 watts. [more]
See the complete analysis at: http://www.macobserver.com/tmo/article/watt_to_know_about_iphone_ipad_power_adapters/