EFI - Extensible Firmware Interface.

Orignally started by Intel for the Itanium cpu (64 bit).
* EFI protocol still owned by Intel
* UEFI open source version or least independent of Intel 
  Independent of cpu.

Replaces or expands legacy BIOS (backwards compatible with original AT).

Provides boot service and runtime services.

Boot time - recognized larger disks, OS independent.
  Boot loader for multi-os systems.
  Supports many more partitions than legacy MBR / 4 partition configuration. 

Run time - drivers independent of OS and even processor.
  Written in EFI Byte Code executed by a cpu specific interpreter.
  Although some cpu specific drivers exist for things such as graphicial
    interace as system comes up.
 
EFI BIOS allows for extensions to expand its functionality.

EFI extensions do not reside at the top of the historical 1 MB memory space but 
  can reside in any non-volatile memory attached to the system.

EFI is NOT an open protocol.

EFI/UEFI may replace legacy bios or it may sit on top and expand functionality.
If on top, it may not be fully active until OS loads.
  Not all features available on boot drive.

EFI/UEFI provides a security feature that can also lock a particular OS on 
  a system unless the system manufacturer provides an optional bios 
  configuration switch.

Most PCs should have this. However, Microsoft has indicated that it won't 
  provide this on their Windows phones (ARM cpus).

Alternatives
   OpenBios
   Coreboot