An external hard drive is a device for storing data that exists outside your computer's built in hard drive. It usually plugs into your computer using a cable, like USB or FireWire. It generally has higher storage capacity than your built in hard drive, sometimes significantly higher. Some external hard drives need to be plugged in, but many are portable and work off of your computer's electricity. There are even wireless drives and drives that work as personal cloud storage.
You may have to shop around for a hard drive that meets your requirements for capacity, quality, and price. Here are some links to reviews of hard drives to help you get started.
Storing files on an external hard drive is a very simple way to save copies of important documents. You can organize files into folders the same way you would on your built in hard drive. You can also set up automatic backups of certain folders or your entire hard drive on your external drive (more on this under the Data Backup Services tab above).
External hard drives are also increasingly cheap ways to backup files. You can generally purchase a terabyte of storage for under $100. To give you a sense of how much space that is:
Here are some considerations that you should think about when choosing an external hard drive: