A boot sector is the sector of a persistent data storage device (e.g., hard disk, floppy disk, optical disc, etc.) which contains machine code to be loaded into random-access memory (RAM) and then executed by a computer system's built-in firmware (e.g., the BIOS).

Usually, the first sector of the hard disk is the boot sector, regardless of sector size (512 or 4096 bytes) and partitioning flavor (MBR or GPT).

The purpose of defining one particular sector as the boot sector is inter-operability between firmware and various operating systems.

The purpose of chain-loading, first firmware (e.g., the BIOS), then code in the boot sector, and then, for example, an operating system, is maximal flexibility.

On an IBM PC compatible machine, the BIOS selects a boot device, then copies the first sector from the device (which may be a MBR, VBR or any executable code), into physical memory at memory address 0x7C00. On other systems, the process may be quite different.

The UEFI (not legacy boot via CSM) does not rely on boot sectors, UEFI system loads the boot loader (EFI application file in USB disk or in the EFI system partition) directly. Additionally, the UEFI specification also contains "secure boot", which basically wants the UEFI code to be digitally signed.

In case a boot sector receives physical damage, the hard disk will no longer be bootable, unless used with a custom BIOS that defines a non-damaged sector as the boot sector. However, since the very first sector additionally contains data regarding the partitioning of the hard disk, the hard disk will become entirely unusable except when used in conjunction with custom software.

A disk can be partitioned into multiple partitions and, on conventional systems, it is expected to be. There are two definitions on how to store the information regarding the partitioning: