The bootloader is a piece of software installed on the hard disk of the computer, that manages the system startup. If on the computer are installed more operative systems (for example Openmamba and Windows XP), normally the boot loader provides a menu that allow to choose which one start.
The bootloader used by openmamba is GRUB 2 that's automatically installed by the mamba installer. On the PC that uses a startup like MBR (Master Boot Record), it's normally possible to start a system that is installed in one of the first 4 partitions of the disk identified by the BIOS as the first hard disk. For the Mac-intel the situation is different because they uses the EFI (Extensible Firmware Interface) startup.
To manage a special boot situation (installation of openmamba on a extended partition, or on a secondary drive), is advised to install the boot loader on the system partition (instead of the MBR) and eventually install the GAG boot loader in the MBR.