UEFI

Unified Extensible Firmware Interface A standard programming interface for setting up and booting a computer from the UEFI Forum (www.uefi.org). It evolved from the EFI interface developed by Intel, which was first used in Intel's Itanium line. UEFI also provides runtime services, which are used to access the drivers that were loaded in the pre-boot environment. For example, when booting from the network, the operating system can continue to use the same network protocols used to download it in the first place.
The Legacy BIOS
The BIOS dates back to the IBM PC in 1981, when the machine operated in 16-bit mode, and BIOS code was jammed into unused blocks within the first 1MB of memory. BIOS enhancements remain limited to the original 16-bit architecture (see PC memory map).
BIOS Compatibility
UEFI provides compatibility with legacy PCs by providing BIOS compatibility. UEFI-enabled PCs use UEFI routines to perform the test and configuration phases and then load a compatibility system module (CSM) that sets up legacy BIOS interfaces (interrupts) for use by the system software. In 2006, more than 12 million PCs shipped with EFI/UEFI booting and the CSM, but without making the EFI/UEFI interface available. See BIOS.
Windows support for UEFI started with the 64-bit versions of Windows Server 2008 and Windows Vista Service Pack 1 (SP1). In addition, Apple's Intel-based Macs are also EFI/UEFI compliant.