OK, USB3 is faster than USB2, so if u get a USB pen drive or USB external drive, the transfer speeds are much higher with USB3. USB3 is blue.
If u don't have USB3 device and u will use USB3 port, it is backwards compatible, but it will use the speed of the lowest device plugged in.
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Here is the detailed info @ http://www.techrepublic.com/blog/10things/10-things-you-should-know-about-usb-20-and-30/1265