To add to Michael's post the higher USB numbers are backwards compatible on machines with a lower USB number, but not vice versa. IE A USB3 drive will run as if its a USB2 device on a machine with only USB2.
Also look closely at machine specs (if you are buying a machine with USB3), not all USB ports on the machine will necessarily be USB3. Maybe only one or two will be USB3 while the rest are USB2.
Also if you use a USB2 hub plugged into a USB3 port then all USB3 devices plugged into the hub will only run as USB2 and not USB3.
It's obvious to some people, but not to others.
