PowerPCs are a quite large family of computers. They start with small 32bit laptops and older Macintosh computers and find their end in IBM's fastest RS/6000 ("RISC SYSTEM") boxes.
Apple uses OpenFirmware.
The so called OldWorld Macs have a part of their OS in this ROM.
All the systems with translucent colored plastic cases (iMac and later) and most of the PowerBooks produced 1999 and later are so called NewWorld Macs.
To find out more about OpenFirmware follow the links in the firmware section of this document.
Macs with an OldWorld Open Firmware ROM include a part of MacOS in this ROM. This Firmware is not upgradeable by any Software.
The easiest way to boot Linux on these boxes is a small bootloader called BootX which is started from a running MacOS < 10.
There's no bootloader I know of, that boots Linux from MacOS X like BootX does from MacOS < 10. BootX also will not work in the classic environment of MacOS X.
Another way is using quik which seems quite tricky to setup and rendered installed systems (nearly) useless after some unsuccessfull trials installing it.
Macs with a NewWorld OpenFirmware cannot use BootX or quik.
On these machines yaBoot is the needed bootloader. It cannot only boot Linux, but also MacOS (X), directly. Furthermore it is able to boot from CD-ROM and even network.
For more details about configuring yaBoot have a look into the yaBoot HowTo.
RS/6000 don't have a real bootloader yet. You just keep some free space in front of your first partition and cat or dd the kernel image directly to the HDD's first sectors. Also, kernel command line options are directly compiled into the kernel image (but can be changed with a spartanic in-kernel command line editor).
However, http://penguinppc.org/projects/yaboot/ states that it should also work on RS/6000 machines. I've not seen that yet, but it might work, though.