BackupA backup is simply a copy of the data you don't want to lose, on a device that will not be affected by the loss or failure of the computer.
Why should you have a backup? Because you don't want that information to get lost! The most fragile piece of equipment in the computer is the one that stores your data. That's the hard drive. It can quit working at any time. If your hard drive is more than 5 years old, it is already on borrowed time.
Examples of data you might back up are: pictures, documents, music, Internet Favorites, and financial records. If you use an email program like Outlook (rather than a website), you will also want to back up your email messages and addresses.
Most people have no need to back up the entire system, by which I mean the nuts and bolts of Windows and its programs. That stuff can be replaced by reinstalling Windows or buying a new computer, but your pictures and documents are irreplaceable.
The easiest way to make a backup is with an external hard drive. They cost between $50 and $150, but for most people it is not necessary to spend more than $100 for a very high capacity drive. A USB flash drive can be used for a smaller amount of data, and they are often under $20. CDs and DVDs can be used as well, but most people don't find them easy enough to actually get around to doing it.
Backup programs will often come factory-installed on an external hard drive, but I have never seen a backup program that I like. I use my own backup program, which can be modified to back up anyone's data. I can set it up for you if you would like. It first copies all your stuff to the external drive, then each time you click on it afterward, it only copies things that are new or changed since the last backup. It's easy to update with a click, and everything ends up in a form that is easy to recognize and easy to use.
Online backup services are also handy, especially Carbonite. I recommend having first a local backup, then if you'd like, an off-site backup for extra protection.