While Windows has had support for the Unified Extensible Firmware Interface (UEFI) prior to Windows 8, most consumer computers have continued to boot using BIOS firmware.
This changes with Windows 8, as UEFI firmware mode is now a Windows 8 client logo requirement. UEFI is also required to enable several features
and improvements, such as:
GUID Partition Table (GPT) disk partitioning - GPT partitions enable larger partitions,
and are supported by UEFI.
Boot from large disk drives - GPT and native 4K sector disk support
in Windows 8 enables support for >2.2 TB boot drives.
Secure Boot - Signature checks on early boot components, helping to protect pre-boot manager components from tampering.
Measured Boot - Works with the TPM to log startup components and activities.
Early Launch Anti-Malware - Registering and loading an anti-malware
driver as a trusted boot-critical driver to help protect the system earlier in the
boot process than with previous architectures.
Trusted Boot - A combination of Secure Boot, Measured Boot and Early Launch Anti-Malware that helps establish
that the system is in a trusted state.
Boot on Computer with no VGA Support - UEFI removes the need for VGA support, enabling Windows 8 to be installed on computers that do not use this legacy video
technology.
Some of these features are targeted for use in a business setting, so they are not covered in this consumer-focused training. Instead, we will examine UEFI support in general, and any support considerations for UEFI enabled Computers in a consumer setting.
There are a lot more specific details here, if anyone wants to go deeper:
http://msdn.microsoft.com/en-us/windows/hardware/gg463386