The nice people developing USB 3.0 knew that compatibility with USB 2.0 was going to be needed so they designed USB 3 with this in mind.
New USB 3.0 devices can be plugged into existing USB 2.0
New USB 3.0 cable retains compatibility.
Since the new design is physically the same size and shape it can be plugged into existing USB 2.0 ports although you won’t get the speed increase, you can still use the device.
In some cases USB 3.0 devices still work slightly faster in USB 2.0 ports.
Six new wires added
The USB 3.0 cable uses a blue color to differentiate from the old.
The existing wires in the USB cable remain and six new wires have been added to accommodate USB 3.0. These allow both interfaces to work in parallel with each other.
A helpful blue colour on the end of the new USB 3.0 connector lets us know it’s a USB 3.0 cable.