Android System Recovery ( Recovery Mode )
The Android system recovery tool is a feature on Android devices which can allow a person to perform certain functions without having to access its settings or even power it on completely. This includes manually updating the software, clearing the cache partition, restarting it, or even performing a hard reset. ( http://www.smartmobilephonesolutions.com/content/android-system-recovery )
Download Mode ( ODIN mode – Samsung )
Android Download Mode which is a sub-system of your device that can used to flash stock firmware files or any other type of customization file. ( http://www.android.gs )
Hard reset ( Factory Reset )
A hard reset, also known as a factory reset or master reset, is the restoration of a device to the state it was in when it left the factory. All settings, applications and data added by the user are removed. The term is often heard in reference to smartphones and tablets but laptop and desktop computers, as well as most other electronic devices, can usually be restored to factory conditions. ( http://whatis.techtarget.com/definition/hard-reset-factory-reset-master-reset )