The iPad has a powerful battery that requires more energy to charge than smaller devices such as the iPhone. Apple ships a 10W wall charger to insure that charging will be efficient.
The iPad can also be charged by high-power USB 2.0 ports. USB ports not meeting this specification will sync the iPad but cannot charge the device. One example of a USB port not capable of charging the iPad is the keyboard USB port on a Mac.
The iPad might charge slowly on non-high-power USB ports if the device is in sleep mode. Similarly, a 5W iPhone wall charger will slowly charge the iPad.
For the best charging results, use the 10W USB power adapter or locate a high-power USB 2.0 port on your computer.