The Ultimate Answer

Set it, then forget it.

Time Machine works with your Mac and an external hard drive. Just connect the drive and assign it to Time Machine and you’re a step closer to enjoying peace of mind. Time Machine will automatically back up your entire Mac, including system files, applications, accounts, preferences, music, photos, movies, and documents. But what makes Time Machine different from other backup applications is that it not only keeps a spare copy of every file, it remembers how your system looked on a given day — so you can revisit your Mac as it appeared in the past.
iMac with Time Machine

Every change, every hour.

Following the initial backup of your entire Mac, Time Machine automatically makes incremental backups every hour, every day, copying just the files that have changed since your last backup. And it does this all in the background, so you can continue working while Time Machine is busy copying your files. Time Machine saves the hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month.

Go back in time.

Say you accidentally deleted a file you meant to save. Simply enter the Time Machine browser and you’ll see exactly how your computer looked on the dates you’re browsing. You can browse for files using Cover Flow, or you can perform a Spotlight search to find what you need across all your backups. Use the timeline to select a specific date, or let Time Machine fly through time to find your most recent changes. Before recovering a file, use Quick Look to verify the contents of the file, then click Restore to bring it back to the present