Linux on your powermac
Debian is a free or Open Source, operating system (OS) for your computer. It's like a tower. At the base is the kernel. On top of that are all the basic tools. Next is all the software that you run on the computer. At the top of the tower is Debian-carefully organizing and fitting everything so it all works together.
An operating system is the set of basic programs and utilities that make your computer run. At the core of an operating system is the kernel. The kernel is the most fundamental program on the computer and does all the basic housekeeping and lets you start other programs. Debian is kernel independent. It currently uses the Linux kernel but work is in progress to provide Debian for other kernels, using the Hurd.
Linux is a completely free piece of software started by Linus Torvalds and supported by (probably over 1000) programmers worldwide. A large part of the basic tools that fill out the operating system comes from the GNU project; hence the names: GNU/Linux and GNU/Hurd. These tools are also free.
Of course, the thing that people want is application software: programs to help them get what they want to do done, from editing documents to running a business to playing games to writing more software. Debian comes with over 3,950 packages (precompiled software that is bundled up in a nice format for easy installation on your machine)--all of it free.
Making Power Macintosh 8500/120 ready for Debian GNU/Linux
Mac will be running MacOS, the native operating system of Macintosh. Don't destroy the contents in the first shot. If required work with MacOS for some time to know how Mac works. It is essential to know details about Mac before installing Linux on Mac. The knowledge of OpenFirmware is necessary in installing Linux on Macintosh. If there is any valuable data, get a backup.
Getting Debian GNU/Linux