Recovery mode in OS X is a special boot options that allows you to run repair and reinstallation tools for your system, in addition to setting a firmware password, if needed.
Through the development of OS X, these tools have been provided in several different forms, and subsequently several different approaches for loading them.
DVD installation
If your system came with a gray recovery DVD, or if you purchased a retail copy of OS X on DVD, then you can use either of these discs to run recovery routines. To do so, follow these steps:
After doing this, the system may take a while to load the recovery tools, but they should eventually show up. Note that you may need to access some from a Utilities menu in the standard OS X installer application that loads.
Local Recovery
Starting with OS X Lion, Apple introduced a local 650MB recovery partition with standard OS X installations, that contains a recovery boot image. In addition to supporting features like FileVault disk encryption, this partition contains all the tools necessary to reinstall or recover OS X, in addition to features like Safari access for looking up help, if needed. To load local recovery, follow these steps:
When Recovery loads, you will see a tools window with options to reinstall OS X, recover from backup, Disk Utility, and Safari. In addition, tools for firmware passwords and account password resets are available in the Utilities menu.
The recovery drive contains a number of tools for reinstalling OS X, setting firmware passwords, and resetting system account passwords (click for larger view).
Internet Recovery
Starting in 2010, Apple introduced its Internet Recovery service, which allows a system to download the contents of the Recovery HD partition from Apple’s servers, instead of having to load the recovery tools locally. This option allows a system to run these tools in the event its hard drive has been completely wiped or otherwise does not have recovery tools on it.
To load Internet Recovery, you need to be connected to a non-enterprise WPA/WPA2-secured Wi-Fi network, and then follow the steps above for Local Recovery but instead hold Option-Command-R when starting up. When you do this, you will see a spinning globe appear as the system downloads the recovery tools from Apple.
Note that Internet Recovery is only supported on the systems in the following list, and later: