There are many different kinds of external hard drive... I can tell u.. network attached storage, external hard drive, pocket hard drive etc..
But really in actual fact, mainly there are only 2 kinds, portable or desktop only.
Desktop has AC power adaptor and based on 3.5" HDD
Portable only has USB Cable and draws power from your computer directly.
I think the portable one is better less messy.. here is the picture:
Click on below if you are interested to buy one.
Desktop External Hard drive
Portable external hard drive.