Quote Originally Posted by danon
the difference between powered and un-powered usb hub is that with the unpowered one, you put strain on your mobo and psu by taking the +5 volts from the psu itself thru the mobo.
Each USB port on your motherboard can supply upto 500mA @ 5v. Thus an unpowered HUB has at most 500mA available to supply ALL of its devices (including itself). In many cases, individual USB devices use far less than the full 500mA and thus it's possible to use an unpowered HUB with several devices, without any problems. This doesn't put any 'strain' on your motherboard or PSU. If you go over the 500mA limit, your devices may start to operate incorrectly, or not at all.

If you power the hub with an external power supply, then each port on the hub is able to supply the full 500mA to any device plugged into it.