Grub is a boot loader and boot manager for Linux and other Unix-based operating systems. It is the first software that runs when a computer starts, and it loads the kernel of the operating system and then initializes the rest of the operating system. Grub is the most common bootloader for Linux distributions and is used to manage the boot process. It allows users to choose which operating system to boot or select a specific kernel configuration available on a particular operating systems partitions. Grub is also a boot manager, which means it is responsible for displaying the boot menu that allows users to select different operating system kernels to load.

Grub has two versions: Grub Legacy (Grub 1) and Grub 2. Grub 2 was written from scratch and intended to replace its predecessor, and is now used by a majority of Linux distributions. Some of the goals of the Grub 2 project include support for non-x86 platforms, internationalization and localization, non-ASCII characters, dynamic modules, memory management, a scripting mini-language, migrating platform-specific (x86) code to platform-specific modules, and an object-oriented framework.

Grub is not related to lawn grubs, which are the immature form of different scarab beetles and can be a threat to grass.