OS X Mavericks: Back up your Mac
Time Machine keeps a copy of everything on your disk, and saves “snapshots” of files that have changed so you can revisit files as they appeared in the past. These snapshots are made hourly unless you turn Time Machine off. Time Machine stores the snapshots on your computer’s internal disk. If you accidentally delete or change a file, you can use Time Machine to recover it.
You should also back up your files to a location other than your internal disk, such as an external hard disk, a disk on your network, or a Time Capsule. That way, if anything ever happens to your internal disk or to your Mac, you can restore your entire system to another Mac, and get all your information back where it belongs in no time.
The fastest way to start backing up with Time Machine is to attach an external hard disk to your Mac. When you attach the disk and turn it on, a message appears asking if you want to use the disk to back up your Mac. Click Use As Backup Disk, then follow the instructions in Time Machine preferences.