You need to provide some more information on that.
If you are referring to the USB connection of an external drive, here's more on the difference between USB 2.0 and USB 3.0: http://www.diffen.com/difference/USB_2.0_vs_USB_3.0
If you are referring to the form factor, I would say that drives could be 2.5'' and 3.5''. The first ones are laptop drives, while the 3.5'' ones are desktop ones.
You need to provide some more information on that.
If you are referring to the USB connection of an external drive, here's more on the difference between USB 2.0 and USB 3.0: http://www.diffen.com/difference/USB_2.0_vs_USB_3.0
If you are referring to the form factor, I would say that drives could be 2.5'' and 3.5''. The first ones are laptop drives, while the 3.5'' ones are desktop ones.