The Extensible Firmware Interface (EFI) is a specification that defines a software interface between an operating system and platform firmware. EFI is intended as a significantly improved replacement of the old legacy BIOS firmware interface historically used by all IBM PC compatible personal computers. The EFI specification was originally developed by Intel, and is now managed by the Unified EFI Forum and is officially known as Unified EFI (UEFI).
The original motivation for EFI came during early development of the first Intel-HP Itanium systems in the mid-1990s. PC BIOS limitations (16-bit processor mode, 1 MB addressable space, PC AT hardware dependencies, etc.) were seen as clearly unacceptable for the larger server platforms Itanium was targeting. The initial effort to address these concerns was initially called Intel Boot Initiative and was later renamed to EFI.
EFI specification 1.02 was released by Intel on December 12, 2000. (Version 1.01 was the original issue; it had incorrect legal and trademark information and was quickly withdrawn.)
EFI specification 1.10 was released by Intel on December 1, 2002. It included the EFI driver model as well as several minor enhancements to 1.02.
In 2005, Intel contributed this specification to the UEFI Forum, who is now responsible for its development and promotion. EFI was renamed to Unified EFI (UEFI) to reflect this; most documentation uses both terms interchangeably.
The UEFI Forum released version 2.1 of the UEFI specification on January 7, 2007; as of March 2007, it is the latest publicly available specification. It added and improved cryptography, network authentication, and the User Interface Architecture (Human Interface Infrastructure in UEFI).