One thing I didn't seen mentioned in the responses so far is speed. USB 1.1, 2,0 and 3.0 all have different speeds. You can generally connect lower speed devices to high speed USB ports but the overall effect will be to force every other USB device on that port to run at the lower speed. So plugging a USB port expander (hopefully a powered one) into a computer's USB port will give you more USB ports, but plugging something like your mouse into that port expander will cause other devices also connected to it to run a lot slower. Typically things like disk drives, geek sticks, cameras, printers, etc are all capable of running at higher speeds than your mouse or keyboard.