E.2.1. GRUB and the x86 Boot Process
The boot process used by other operating systems may differ. For example, the Microsoft® Windows® operating system, as well as other operating systems, are loaded using chain loading. Under this method, the MBR points to the first sector of the partition holding the operating system, where it finds the files necessary to actually boot that operating system.
GRUB supports both direct and chain loading boot methods, allowing it to boot almost any operating system.