GRUB (GRand Unified Boot Loader) is a flexible boot loader. It supports GNU's multiboot standard, and also chain loading (ie, it can use boot block from partitions; this allows it to boot operating systems such as Windows.)
It also knows about many different partition types, which allows it to read files and kernels from partitions themselves; This means that you can edit your boot menu file while the machine is running without need to reinstall, and that location of the kernel can be changed "on the fly".
It's very pretty, compared to LILO: It uses a pretty character-based menu with configurable colors! I've booted my machine a few times just to see the pretty menu... =)
Also, it's nicer than LILO because with that I have never needed to add any "mem=xxxx" lines - Linux boots and the memory is Out There. And, 0.9x also have very good "grub-floppy" and "grub-install" programs for boot floppy making and hard disk boot block installation - no need to dd files to disks anymore, no need to boot from floppy and install stuff. Basic installation is just as easy as LILO install now, maybe even easier: Make the menu file, Run grub-install with a couple of parameters, and you're set!
Here's also an interesting quote from Gordon Matzigkeit that was taken from the GRUB manual:
Some people like to acknowledge both the operating system and kernel when they talk about their computers, so they might say they use "GNU/Linux" or "GNU/Hurd". Other people seem to think that the kernel is the most important part of the system, so they like to call their GNU operating systems "Linux systems."
I, personally, believe that this is a grave injustice, because the boot loader is the most important software of all. I used to refer to the above systems as either "LILO" or "GRUB" systems.
Unfortunately, nobody ever understood what I was talking about; now I just use the word "GNU" as a pseudonym for GRUB.
So, if you ever hear people talking about their alleged "GNU" systems, remember that they are actually paying homage to the best boot loader around... GRUB!
"Grub first, then ethics." - Bertold Brecht