If you're using custom hardware, or want to replace the ROM monitor on your board, you'll need a boot loader that knows how to decompress and transfer control to the kernel. The current trend is to include this functionality in the ROM monitor, as is the case with ppcboot.
Although some of the data is board specific, you can assemble the bits from information available on the net: http://lists.linuxppc.org/listarcs/linuxppc-embedded/199911/msg00034.html
For PreP and PowerPlus SBC's, see:
The old bootloader included in the kernel source tree, originally written for the Motorola MBX board, and since ported to a wide range of platforms. This is the start-up component of the zImage that knows how to decompress the kernel. mbxboot is now being rendered obsolete by ppcboot.