Generally, an EFI is an interface between the hardware and its firmware and the loader of the operating system. Practically, what a user interested in dual booting or multi booting Windows 8/8.1 and Linux needs to know are that an EFI system can only be used with a GPT disk and the following.
Installing and configuring GRUB for an EFI system is a simple matter of a few commands to make the EFI System partition available, to install the appropriate version of the GRUB package, to install GRUB (the bootloader not the package) in the EFI System partition, and to update the configuration. The steps given here can be used to repair GRUB, to install and configure GRUB for EFI on distributions that don't do it for you …