GRUB
also: GRUB2, GRand Unified Bootloader
GRUB (GRand Unified Bootloader) is the boot loader that loads the Linux kernel into memory when your computer starts up, allowing you to select which operating system or kernel version to boot.
GRUB is the most common bootloader on Linux systems. It runs after the BIOS/UEFI firmware initializes hardware and is responsible for locating, loading, and executing the Linux kernel and initial RAM disk (initramfs).
GRUB displays a menu at boot time, letting you choose between different Linux kernels, operating systems, or boot parameters. For example, you can select a previous kernel version if the latest one has problems, or boot into single-user mode for recovery.
Configuration is typically stored in /boot/grub/grub.cfg on modern systems, though this file is auto-generated from settings in /etc/default/grub and files in /etc/grub.d/. You modify these source files and run grub-mkconfig to rebuild the main config.
GRUB 2 is the modern version, supporting both legacy BIOS and newer UEFI firmware, multiple filesystems, and encrypted partitions. It replaced GRUB Legacy, which is no longer maintained.