UEFI
Introduction
(U)EFI or (Unified) Extensible Firmware Interface is a specification that defines a software interface between the operating system(s) and the platform's firmware. In the mid 90s Intel was creating a new processor architecture that was 64-bit, but wasn't backwards-compatible with the old x86. This architecture was the Itanium 64. Because the IA-64 only supports 64-bit instructions, the PC BIOS couldn't be used, therefore Intel developed the EFI specification. Later on this specification was managed (and still is) by the UEFI board, an association of several companies such as AMD, Microsoft, Intel, Apple and so on.
Support
The (U)EFI specification is currently defined for the Itanium platform, the x86-platform (64-bit included) and the ARM platform. Since Apple's movement with Mac OS X from the PowerPC architecture to the x86-architecture, they have been using EFI, although with minor differences here and there. Most modern operating systems, boot loaders and boot managers support (U)EFI as well nowadays. Most 64-bit versions of Microsoft Windows, FreeBSD, Apple Mac OS X and Linux all have support for UEFI to some extent. As for boot loaders and managers you generally have (E)GRUB and ELILO. As for emulators and virtual machines, you can generally use VirtualBox and Qemu.