Bootloader

GRUB (GNU GRand Unified Bootloader) is the bootloader used by the majority of Linux distributions on x86 and x86-64 systems. Its role is to bridge the gap between what firmware hands control to and what the Linux kernel needs: firmware (BIOS or UEFI) loads GRUB, and GRUB locates the kernel image and initrd on disk, assembles a kernel command line, and transfers control to the kernel. Because GRUB understands a wide range of filesystem formats — ext4, XFS, Btrfs, FAT, and more — it can read its own configuration and the kernel directly from the root or boot partition without any intermediate step, which is what distinguishes it from simpler bootloaders that can only read from FAT.