Computer technology generally falls into two categories: hardware and software. Hardware is anything you can physically touch. This includes the computer itself as well as anything attached to it. Software is the electronic instructions that make your computer perform some task. When you turn your computer on, the first software it loads is the operating system. Without an operating system, your computer would be useless. It would just sit there with a nearly blank screen doing nothing. The operating system is what lets you interact with the hardware. When you type on your keyboard, the operating system turns those keystrokes into letters on the screen. When you save a file, the operating system handles the details of how to store it on the hard disk. Microsoft Windows is the operating system that runs on over 90% of personal computers. There have been many versions of Windows as it has evolved and improved over the years including Windows 3.1, Windows 95, Windows 98, Windows Millennium Edition, Windows NT 4.0, Windows 2000, Windows XP, and Windows Vista.

A computer with just an operating system isn't terribly useful. Application software (also called applications, programs, or just "apps" for short) is software that does something useful for you. Applications include word processors, spreadsheets, music players, web browsers, email clients, accounting packages, photo editors, and thousands more. Applications are written to run on a specific operating system. If you want to run Microsoft Excel for Windows or Intuit QuickBooks for Windows, you need Microsoft Windows as your operating system. When shopping for software, the side of the box or description on the web site will list the specific versions of Windows that program requires. For example, Microsoft Office 2007 will run on Windows XP, Windows Vista, and Windows 7. It will not run on older versions of Windows such as Windows 2000 or Windows 98 and it will not run on other operating systems such as Apple's OS X or Linux.

Since applications are written to run on a particular operating system, it does not matter what brand your computer is as long as you have the right operating system. Dell, HP, Gateway, Toshiba, etc all sell computers with Microsoft Windows so any software requiring Windows will work on them. The only major computer maker not selling Windows computers is Apple. Apple has their own operating system that you can only get on an Apple computer. Since the operating system is different, Windows programs will not run on an Apple computer unless you purchase and install a copy of Windows first.

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