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 very 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 a firmware (e.g., the BIOS), then some code contained in the boot sector, and then, for example, an operating system, is maximal flexibility.
The IBM PC and compatible computers
On an IBM PC compatible machine, the BIOS selects a boot device, then copies the first sector from the device (which may be an MBR, VBR or any executable code), into physical memory at memory address 0x7C00. On other systems, the process may be quite different.
Unified Extensible Firmware Interface (UEFI)
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.
Damage to the boot sector
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.
Partition tables
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:
A master boot record (MBR) is the first sector of a data storage device that has been partitioned. The MBR sector may contain code to locate the active partition and invoke its volume boot record. A volume boot record (VBR) is the first sector of a data storage device that has not been partitioned, or the first sector of an individual partition on a data storage device that has been partitioned. It may contain code to load an operating system (or other standalone program) installed on that device or within that partition.
The presence of an IBM PC compatible boot loader for x86-CPUs in the boot sector is by convention indicated by a two-byte hexadecimal sequence 0x55 0xAA (called the boot sector signature) at the end of the boot sector (offsets 0x1FE and 0x1FF). This signature indicates the presence of at least a dummy boot loader which is safe to be executed, even if it may not be able actually to load an operating system. It does not indicate a particular (or even the presence of) file system or operating system, although some old versions of DOS 3 relied on it in their process to detect FAT-formatted media (newer versions do not). Boot code for other platforms or CPUs should not use this signature, since this may lead to a crash when the BIOS passes execution to the boot sector assuming that it contains valid executable code. Nevertheless, some media for other platforms erroneously contain the signature, anyway, rendering this check not 100% reliable in practice.