This program is built into your Mac and pretty much runs with little intervention from you! When you connect your external hard drive, it’s very likely that the Apple Time Machine application will launch and ask you if you want to use that hard drive for a Time Machine backup.
If you choose the “Use as backup disk” option, the application will back up your entire hard drive including all your documents, media files, etc. so that first run will take a good amount of time so you should do it when you don’t need to use your laptop (you should make sure your laptop is plugged in). The icon on your top menu bar will show the activity as it backs up with the clock’s “hands” going around. Once it’s completed, you’ll see the icon become still and you will be able to eject the hard drive by dragging the hard drive’s icon on your desktop to the Trash or highlighting it and either pressing Command-E or selecting Eject in the File menu while in Finder.
Time Machine will then perform an incremental back up (meaning it will only back up files that have been added or changed) each time you attach that same hard drive to your Mac. If you forget? It will occasionally tell you how long it’s been since you’ve performed a backup. One thing to note though: it can only back up to the capacity of your hard drive after which it will overwrite the older files. For more information on Apple’s Time Machine,visit the knowledge base article on Apple’s site at: http://support.apple.com/kb/HT1427