Time Machine is the fabulous backup application that’s part of Mac OS X Leopard. To use Time Machine, simply connect an external hard drive to your computer. The first time you connect it to your Mac, Leopard displays a dialog asking if you’d like to use it as your backup location. If the dialog doesn’t appear, don’t worry:
Time Machine toggles on, changes the image for your backup disk, and indicates when the next (in this case, the first) backup will occur). For best results, you should use a drive that’s at least as large as your Mac startup drive. (If your internal hard drive can hold up to 250GB of data, your Time Machine volume should hold at least 250GB.) And it’s a good idea not to store any other data on your Time Machine drive. That way, you maximize the amount of space you have to backup your files.