Boot Managers are small applications which allow a user to boot a computer into an operating system, or one of a selection of operating systems.
GNU GRUB is a multiboot boot-loader software package from the GNU project.
LILO (LInux LOader) is a boot loader for Linux developed by John Coffman.
The Linux boot loader loadlin starts Linux from DOS or Win9x.
NTLDR (abbreviation of NT Loader) is the boot loader for Windows NT.