Ditto to that. NAS = network attached storage. The main advantages of NAS unit are that they support redundant/resilient storage configurations (so if you lose a disk you don't lose your data), they are accessible from anywhere on your home LAN and usually provide a whole host of add-on features such as iTunes/multimedia server, remote access, disk encryption, BitTorrent client etc. They are more expensive than a basic external hard drive, but it's a worthwhile investment if you care about the data you are storing.
I pi55ed about with external hard drives for years before switching to NAS. Currently have a QNAP TS-212 at home and we use Synology DS413s at work. Both are excellent.