Restoring

• Restoring the iPhone is sometimes necessary when the device stops functioning properly. If the phone continually lags on loading applications and the problem isn't solved with a simple reboot, then you may choose to restore the iPhone to its original factory settings.
  
  

Upgrading

• The iPhone will go into a subtle, self-directed recovery mode on its own when you choose to upgrade the firmware via the iTunes desktop client. This mode is the same as restoring directly from iTunes, but often you don't have to manually put the device into recovery mode.
  
  

Device Firmware Update

• You should only use the advanced DFU mode of recovery if you are planning to load a modified iOS firmware, common with jailbroken iPhones. Another reason to use the DFU mode would be to downgrade the firmware from a current version to a previous one. Downgrading might be the only choice if a particular firmware version causes severe technical glitches. You might downgrade, for example, when you have restored the iPhone from recovery mode but find that the action still didn't resolve the issue.
  
  

How to Use Recovery Mode

• To place your iPhone into recovery mode, launch the iTunes software on your computer. Press and hold the "Home" and "Power" buttons simultaneously to power off the device. Release the "Power" button once the screen turns black and continue to hold the "Home" button down. After a few more seconds, the iTunes icon will appear on the screen with a picture of the USB cable. Once you see these graphics, connect the iPhone to the computer via the cable and wait for the instructions provided by the iTunes software.