UEFI (Unified Extensible Firmware Interface) is a boot level program that will replace the BIOS on computers made from this date forward. UEFI defines the software interface between the operating system and platform firmware. It resides in non-volatile memory on the motherboard (or on the hard drive or a network drive) and initializes the computer to run. Designed as a replacement for the BIOS, it is less like the current BIOS (Basic Input/Output System) and more like an operating system. For instance, UEFI provides a GUI interface for operators and can run commands from a shell.
UEFI allows developers to design additional functionality into the system that will fire off before the operating system is loaded. Examples of new tasks that could be introduced include backing up hard drives, running utility programs, or logging into a secure network. Its inherent Internet capabilities allow for remote diagnostics and repair of computers even without an operating system installed.
Whereas BIOS was 16-bit, UEFI can be 32-bit or 64-bit (and the operating system must be sized match to the UEFI implementation being used).