A boot sector is a specially assigned section of a storage drive containing the files required to start the operating system (OS) and other bootable programs. Bootable programs include some antivirus programs, drive partitioning software, backup tools and diagnostic disks as well as standard operating systems.
Initially when a computer is started, the basic input output system (BIOS), extensible firmware interface (EFI) or unified extensible firmware interface (UEFI) launches the boot loader of the assigned device or the first of a preference-ordered list of boot devices. Often this process involves copying the bootloader into faster RAM first.
On a PC-compatible platform or a storage device that has not been partitioned or logically broken up into a number of storage volumes, there is generally only one type of boot sector, a master boot record (MBR). This is the first sector on the drive. On systems that are partitioned the first sector of each volume contains a volume boot record (VBR).
While newer systems such as EFI and UEFI can run as a lightweight OS and launch shell programs without the use of a boot sector, standard operating systems continue to use this method of launching. Corruption of the boot sector is a common cause of startup issues. Boot sectors are often an attack vector as well because they launch code automatically. Hackers and boot sector viruses sometimes try to inject malicious code that will run prior to the launch of the many security layers.