Photograph by Robert Cardin
The Unified Extensible Firmware Interface is a new firmware interface specification that is designed to replace the familiar BIOS interface.
When you turn on your computer, UEFI's firmware will run an inventory of the hardware installed on the system; after checking that everything is functioning properly, it will launch the operating system and turn control of the PC hardware over to the software (and to you). UEFI supports a wider range of chip architectures (including 32-bit and 64-bit processors like the ARM chips that will be in Windows 8 tablet PCs) 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 you should never have a problem upgrading from a motherboard flashed with BIOS to one flashed with UEFI.