External drives come in a variety of shapes and sizes. If you're looking at using this drive as a permanent extention that is almost always used when the computer is used, it's probably a good idea to look at the faster interfaces.
The order or speed in external drives is, USB as the slowest, then Firewire, and the fastest being eSata.
When you look at buying an external drive, there are a couple of things to consider.
First, ask yourself what is more important, speed or pricing. If speed is your number one concern, get an eSata drive. If it is pricing get a USB drive.
Second, determine if you are just buying a external drive, or building one. If you are building one, buy the hard drive just like you would when building a computer. Then pick out an enclosure for it, and assemble it according to the instructions that came with the enclosure.
Make sure you check the reviews. If you're buying a drive to put in and enclosure, check the benchmarks section on how to build a computer page.
Third, if you're looking at this storage for a portable device, like a laptop, make sure you buy an external drive that requires no extra power.