High-power usb port

Q2: What's the deal with low/high power devices and bus/self powered hubs?
A2: You may connect high power devices to self powered hubs only and low power devices to either bus or self powered hubs.
High power devices are devices that draw more than 100mA from the USB power line, low power devices are ones that draw 100mA or less. High power devices are typically bus-powered cameras, bus powered hubs. Low-power devices are typically mice, keyboards, joysticks, and any devices that come with their own power supply (also known as self powered devices). Most general purpose hubs come with their own power supply as well and therefore are self powered. A subset of hubs called bus powered hubs gets power from the bus and therefore has the limitation of only supporting low power devices.

Courtesy of this site:
http://www.usb.org/about/faq/ans3