Many web sites simply state that the iPad will charge, but more slowly, wpith the small 5W iPhone chargers.
Apple says the same thing here, but in the table on the same page, lists only the iPad mini.
The Belkin Micro Auto charger I have is apparently 5W, like the Apple iPhone charger. Belkin makes 10W car chargers as well and dual 10W car chargers. (I may get ordered one of those. We needed to charge two iPhones on a recent car trip.)iPad is designed to charge with the USB Power Adapter provided in the box. iPad will charge with any Apple USB Power Adapter and will also charge, although more slowly, when attached to a computer with a high-power USB port. When attached to a computer through a standard USB port (most PCs or earlier Mac computers) iPad will charge, but only when it's in sleep mode. Make sure your computer is on while charging iPad through USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Some dual chargers are 5W on one USB port and 10W on the other.
The latest Apple iPad chargers are 12W.
So it looks like it's all about speed of charging.