http://en.wikipedia.org/wiki/USB
Power
The USB specification provides a 5 V (volts) supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines.[10] Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 2 mA up to a maximum of 500 mA.
If a bus-powered hub is used, the devices downstream may only use a total of four units — 400 mA (i.e. 2 watts) — of current. This limits compliant bus-powered hubs to 4 ports. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available.
On-The-Go and Battery Charging Specification both add new powering modes to the USB specification. The latter specification allows USB devices to draw up to 1.5 A from hubs and hosts that follow the Battery Charging Specification.
As of June 14, 2007, all new mobile phones applying for license in China are required to adopt the USB port as a power port.[11]
In September, 2007 the Open Mobile Terminal Platform --a forum dominated by operators but including manufacturers such as Nokia, Samsung, Motorola, Sony Ericsson and LG--announced that its members had agreed on micro-USB as the future common connector for mobile devices. [12][13]