The hard drive is the device in a computer which holds most of your important information.
Formerly known as the 'secondary memory', together with tape drives and floppy disks, they are now generally considered to be an essential component of a computer system. Hard drive capacity has grown from 20Mb (1986) to around 300Gb (2006). Even for economy models, hard drives now hold about 350 times the data, at about the same price, as they did some ten years ago.
Hard drives can be used as portable media, but don't drop them, they'll break.
If someone tells you, 'Your hard drive has crashed' or 'It's getting bad clusters', take out your handkerchief and prepare to cry, because it probably means that a lot of your important documents and letters are lost forever, and that you'll have to get a new one.