GRUB is the GRand Unified Bootloader.
There are two major flavors:
- GRUB, the legacy version developed by Erich Boleyn, bearing version numbers up to and including 0.97.
- GNU GRUB, also known as GRUB 2, is the currently-developing version, bearing version numbers from 1.90 (August, 2005) and upwards.
The two do not interoperate, and use very different configuration methodologies. However, the appearance on bootup is similar enough to not confuse the ordinary user.
As of May 2011, GRUB 2 is about 6 years old, but has not yet been adopted by all Linux distributions. Even Ubuntu, which has adopted GRUB 2 around 2010, uses plain "GRUB" to refer to the legacy version. It may happen over time that the usage of legacy GRUB will become rare and plain "GRUB" will be taken to refer to GNU GRUB, perhaps when there is a version 2.0. As it is, the nomenclature and numbering is already a bit confusing.
- lilo -- the LInux LOader.