The WD Passport is a small portable Hard disk drive that normally goes up to around 1Terabyte.
The WD Element uses a regular sized desktop Hard Disk drive that normally goes up to around 3Terabytes. I believe this version requires an extra power source /and or 2 USB cables to power it (with a split end).
I have the WD Passport - 1TB edition and it is nice and small. USB2.0. Works really well too, and only needs 1 USB cable.