Apple iMac (21.5-inch, Late 2011) - Manual (Page 45)

45 Chapter 4 Problem, Meet Solution
Use the Mac OS X Utilities application to:
 Restore your software and data from a Time Machine backup.
 Reinstall Mac OS X and Apple applications.
 Restore your computer to its factory settings by erasing its disk and reinstalling
Mac OS X and your Apple applications.
 Repair your computer’s disk using Disk Utility.
If your computer detects a problem, it opens the Mac OS X Utilities application
automatically. You can also open the application manually by restarting your computer.