3 Hidden Modes in Android Mobile phones

Fast Boot mode
Download mode
Recovery mode

DISCLAIMER: This information is intended for experienced users. Please do not try any of following methods if you are not familiar with mobile phones. We'll not be responsible for the use or misuse of this information, including loss of data or hardware damage. So use it at your own risk.

1. Fast Boot Mode
This mode is used to flash the phone firmware using command line tools. To access this mode:
Power off your Phone.
Press and hold Call and End Call/Power keys.

2. Download Mode
This mode is also used to flash the phone firmware. Mostly this mode is used by GUI tools for easier and quicker flashing. To access this mode:
Power off your phone.
Press and hold Volume Down, OK and End Call/Power keys.

3. Recovery Mode
This mode is used for recovery purposes like to reset the phone firmware. To access this mode:
Power off your phone.
Press and hold Volume Down, Call and End Call/Power keys.

Once the alert triangle is shown on screen, press "Menu" key to reset the firmware or press "Home" and "End Call/Power" keys to show recovery menu.