A hard drive lets the user do many things:

It stores the operating system. The first thing your computer does when you turn it on is load the operating system from your hard disk: That's why you hear the drive clicking furiously for a minute or two. The operating system is the basic set of instructions that tells your computer how to launch programs and how to communicate within itself and with the outside world through your disk drives, your monitor, keyboard, printer and other peripherals.

It stores programs.The hard drive is where you store the programs you use every day. Microsoft Word, Lotus 1-2-3 , Quicken, Solitaire, Photoshop and other software packages reside on your hard drive. Even CD-ROM disks rely on your hard drive to store programs and other information they need to access quickly. When you "install" a new program that comes on a floppy disk or CD-ROM, you're actually copying all or part of the program to your hard drive. When you run a program, the operating system reads it from your hard drive and loads it into memory.

It stores your data. The information you create with your programs is stored permanently on the hard drive -- unless you erase it or a glitch turns it into a scrambled mess. When you write a letter to Aunt Thelma with your word processor or create a spreadsheet and "save" it, you're storing it on your hard disk. Your hard drive becomes the main depository of your business records if you use your computer for this purpose.

It enhances your computer's memory. If you use Windows, OS/2 or a Macintosh, your operating system can substitute part of your hard disk for regular memory (or RAM) when real memory runs low. In computerspeak, this is known as "virtual memory." While virtual memory is much slower than the real thing, it lets you run more programs at once than you'd be able to do otherwise.

All of these responsibilities put a real strain on your hard disk, and you'll often hear it churning in the background as it keeps house -- even when you're not doing anything with your computer. In the end, the faster your hard drive runs, the faster your computer performs. So buy the biggest and fastest hard drive you can afford.