Since the 5v rail in most computers is fairly limited in capacity, quite a few USB hubs have their own wall wart to supply power for the hub, and to power any USB devices that feed off of them.

With "separately powered" hubs, you may need to unplug the hub power (in addition to full power off for the computer) to get a full reset of the hub.

It's handy to have a single (or few) multi-tap power strip(s) that all of the warts plug into, so that with the computer down, one switch can cycle them all off and back on.

If you actually use more than 2 or 3 USB devices (at the same time), you probably should have a powered hub, since the stuff you plug into the hub otherwise can overload the computer's 5v line. "Passive" hubs, that use computer power only, are fine if they're just for things like the camera that you plug in only during downloads, etc, where it's just a matter of getting the connection point out from behind the computer, or if you just want to leave the cords with a few of those 7 "special USB connectors" in place but usually "empty" to plug the camera, videocam, etc in for downloads one at a time.

John