Bootloader

A bootloader is a program that initializes a computer enough to load or start an operating system or another program. It bridges early hardware startup and execution of the chosen software.

BIOS: Firmware that initializes basic hardware and selects a bootable device on legacy PCs. On legacy PCs, BIOS performs early startup before handing control to boot code.
Power-on self-test: Firmware checks performed after power-on to detect and initialize essential hardware. These checks precede the firmware's attempt to locate and launch boot software.
GNU GRUB: A widely used bootloader that can start multiple operating systems and pass options to their kernels. GRUB presents boot choices and loads kernels on many Linux systems.
Firmware: Software embedded in a device that provides low-level control of its hardware. Firmware usually runs before a bootloader and may itself launch one.
UEFI: A modern firmware interface that initializes hardware and launches EFI applications, including operating-system loaders. UEFI can load an operating-system loader directly from an EFI System Partition.
Boot sector: A disk sector reserved for startup code or boot-related data. Legacy firmware starts booting from a designated sector on a selected device.
Windows Boot Manager: Microsoft's boot manager, which selects and starts Windows installations or other configured boot applications. It coordinates startup choices and launches Windows operating-system loaders.
Operating system: System software that manages computer hardware and provides services for applications. Unlike an operating system, a bootloader prepares the system to start one.
Master boot record: The first sector of a partitioned storage device, containing legacy boot code and partition information. Legacy bootloaders often begin in the MBR's small initial code area.
Partition table: A data structure describing the layout and boundaries of partitions on a storage device. Boot code or firmware uses partition information to locate bootable filesystems.

Linked from 8 pages