Old World Macs
A bit of history
Apple changed its hardware architecture in the late 1990s, creating a split in its product lines between "OldWorld" and "NewWorld" PowerPCs. Search for those terms for more about the details of the change. This comes up a lot in the Linux world, because installing Linux on a NewWorld machine is generally easy, while installing it on an OldWorld machine is exceedingly difficult, long, or both.
All G4s and later machines are NewWorld. Almost all G3s are NewWorld, including all iMacs and iBooks. However, the "beige" G3 desktop and some early G3 servers are OldWorld. But the most confusing product line is the PowerBook G3 series, which underwent a radical hardware revision while the external design remained virtually unchanged. If you have a G3 PowerBook, you need to determine which side of the divide it's on. The WallStreet and "MainStreet" machines are OldWorld. The Lombard (bronze keyboard) and all later models (e.g. the Pismo) are NewWorld. Try Googling your machine's model number if you're unsure where it falls in that series.
Installing Ubuntu on OldWorld Macs
If your Mac is indeed an OldWorld, and you're determined to install Linux (perhaps because no current Apple OS is supported on it), you're officially out of luck with Ubuntu; it's not officially supported on OldWorld PPCs and any newer versions than Dapper Drake (6.06.1) may not install. However, you can follow the unofficial instructions on this site if you're feeling lucky. Just be aware that it will take a substantial effort on your part, and it may not work.