Multiboot (see Multiboot Specification) is the native format supported by GRUB. For the sake of convenience, there is also support for Linux, FreeBSD, NetBSD and OpenBSD. If you want to boot other operating systems, you will have to chain-load them (see Chain-loading).
Generally, GRUB can boot any Multiboot-compliant OS in the following steps:
Linux, FreeBSD, NetBSD and OpenBSD can be booted in a similar manner. You load a kernel image with the command kernel and then run the command boot. If the kernel requires some parameters, just append the parameters to kernel, after the file name of the kernel. Also, please refer to OS-specific notes, for information on your OS-specific issues.