USBFacts - Knowing Connectivity

The USB specification provides a 5 V 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.
A unit load is defined as 100mA in USB 2.0, and was raised to 150mA in USB 3.0. A maximum of 5 unit loads can be drawn from a port in USB 2.0, and was raised to 6 in USB 3.0. There are two types of devices: low-power and high-power. Low-power devices draw at most 1 unit load, with minimum operating voltage of 4.4V in USB 2.0, and 4V in USB 3.0. High-power devices draw the maximum number of unit loads supported by the standard. All devices default as low-power but the device's software may request high-power as long as the power is available on the providing bus.

A bus-powered hub is initialized at 1 unit load and transitions to maximum unit loads after hub configuration is obtained. Any device connected to the hub will draw 1 unit load regardless of the current draw of devices connected to other ports of the hub (i.e 1 device connected on a 4 port hub will only draw 1 unit load despite the fact that all unit loads are being supplied to the hub).
A self-powered hub will supply maximum supported unit loads to any device connected to it. A battery powered hub may supply maximum unit loads to port. In addition, the VBUS will supply 1 unit load upstream for communication if parts of the Hub are powered down.
In Battery Charging Specification, new powering modes are added to the USB specification. A host or hub charger can supply maximum 1.5A when communicating at low-speed or full-speed, maximum 900mA when communicating at hi-speed, no upper current limit when no communication is taking place. Dedicated charger can supply maximum 1.5A of current. A portable device can draw up to 1.8A of from a dedicated charger. The dedicated charger shorts the D+ and D- pins together and will not send or receive any information on those lines, allowing for the creation of very simple, high current chargers to be manufactured. The increased current (faster charging) will occur once the host/hub and devices both support the new charging specification.
As of June 14, 2007, all new mobile phones applying for a license in China are required to use the USB port as a power port.
In September, 2007 the Open Mobile Terminal Platform—a forum dominated by mobile network 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.