What's the difference in external drives?

I'm looking for an external drive to back up some files as a precaution. Looking at the big box stores I see what look like big price ranges for drives of the same size ($100+ differences). Are there particular features I should be looking for? Some say that they have usb ports, which I would want, but otherwise. Why the price range?

Also, would I need to install any kind of an operating system on the other drive, or does it work just like a jump drive?

Thanks for your help.