A USB port on a computer is isolated/regulated. The USB spec tells you how much current you can draw (which I don't remember... maybe about 1/2 an amp... and it depends on if it's USB 1, 2, or 3... I'm sure it's in Wikipedia, if you want to look it up.)
There are powed hubs like this
. A powered hub has it's own separate power supply. With a powered hub, each USB port can supply the current specified in the USB spec.
An un-powered (or self-powered) hub gets its power from the USB port it's plugged into. Of course in that case, the total power (current) cannot cannot exceed what the one main port can supply.