From LQWiki

EFI or Extensible Frimware Interface is a replacement for BIOS which was created for the Itanium series of processors (ia-64). It runs on several Intel processors, including Itanium, Itanium2, Pentium and XScale/PXA.

EFI systems use GPT partinioned disks in stead of MBR partitioned disks. On linux systems, elilo is used to boot the operating system instead of lilo or Grub.

EFI includes it own shell for system administration even before an operating system is installed.

EFI is supported by most linux variants; including Debian, Red Hat, SUSE and Gentoo. FreeBSD also supports EFI. MAC OS X supports EFI. Microsoft only supports EFI on 64-bit platforms. Apple Boot Camp in fact is a workaround to enable 32-bit versions of MS Windows to work using EFI.

Originally, EFI was created by Intel. Now the EFI standards are created by the Unified EFI Forum (UEFI). Some components of EFI have been open sourced in TianoCore.

External links:

 The Unified EFI Forum: http://www.uefi.org/
 TianoCore open source components: https://www.tianocore.org/
 The EFI shell: https://efi-shell.tianocore.org/