Briefly, a bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel software (such as the Hurd or the Linux). The kernel, in turn, initializes the rest of the operating system (e.g. GNU).
List of bootloaders
|Bootloaders||x86 (IA32)||ARM / XScale||MIPS||PowerPC||SuperH||remarks|
|coreboot, formerly known as LinuxBIOS||yes||-||-||-||-||Conference talk by Peter Stuge at Embedded Linux Conference Europe 2008, video|
A dash '-' means: not researched...
A lot more exhaustive list is available at Wikipedia