Master boot record
When a disk uses a MS-DOS partition table, the first 512 bytes of the disk contain the MBR (Master Boot Record).
Amongst other things, the MBR contains the boot code to bootstrap the bootloader, and the partition table. The first 440 bytes of the master boot record contain the boot code.
Getting rid of GRUB/Lilo/whatever
Most of the time, there is no reason to “remove” the bootloader, as the machine won’t boot anything without one. It is a common practice to replace to bootcode with one of another bootloader.
However, it might sometimes be interesting to get rid of it (for example when it is installed at multiple locations and is therefore redundant).
In this case, to get rid of grub, lilo, or any previously installed bootloader, a quick & easy way is to override it with zeros :
dd if=/dev/zero of=/dev/sda bs=440 count=1