What is UEFI?

What is UEFI, I mentioned it at What is GPT, but not clear. Here I would like to give more details about it.
The Unified Extensible Firmware Interface, UEFI, is a new firmware interface specification that is designed to replace the familiar BIOS interface.
When turn on the computer, UEFI’s firmware will run an inventory of the hardware installed on the system; after checking that everythinkg is functioning properly, it will launch the operation system and turn control of the PC hardware over to the software. UEFI supports a wider range of chip architectures, including 32-bit and 64-bit processors like the ARM chips, than does BIOS, which is limited to running on 16-bit processors.
The new spec works very well, and nearly all UEFI firmware images include support for older BIOS services, so it has no problem upgrading from a motherboard flashed with BIOS to one flashed with UEFI.