boot loader, also referred as boot manager is program that runs in the MBR. Its main purpose is to place the Operating System of a computer into memory.
When a computer is started/powered up, the BIOS performs a few tests and then it transfers the control to MBR in which the boot loader resides. In case of Linux, there are two most common boot loaders, and they are LILO(Linux Loader) and LOADLIN (LOAD Linux). LILO is known for its fast boot up, hence it is one of the most used boot loader. In case of computers that use multiple operating systems, LOADLIN is used, it is also used as a backup bootloader if the LILO fails. In Red Hat, another form of bootloader called as GRUB is used.