Tablets have larger batteries than phones and may be unable to sufficiently charge from the half amp limit of a USB port, especially while they are fully on (screen on and full speed processor clock).
Some will barely charge if the screen is off but come with high current 5v "usb-like" wall chargers, others use a higher voltage charger on a custom connector - typically something round. Sometimes the "usb-like" ones are actually special cables with extra pins that tell the tablet that a special high-current charger is being used.
The Samsung Galaxy Tab has a loading current of 2 A, but the USB-Specificication only allows 500 mA for the whole bus.
Battery Charging Specification 1.2 released in December 2010. Several changes and increasing limits including allowing 1.5 A on charging ports for unconfigured devices, allowing High Speed communication while supplying a current up to 1.5 A, and allowing a maximum current of 5 A
So, What kind of USB Cable should be used to Charge Samsung Galaxy Tablets, for example Samsung Galaxy Tablet 2 (GT - P5100)