Apple has just made final macOS Sierra 10.12 download available to all after months of developer and public testing, meaning that compatible Mac owners can get their hands on it right now.
What is Mac OS X for PPC (OSX 10.0, 10.1, 10.2, 10.3, 10.4, 10.5)? Mac OS X is an operating system by Apple Inc. It is a conglomoration of NeXTSTEP, A/UX, and Mac OS 9. It is based off the unix kernel and provides compatibility with X11. It features a new theme called Aqua which replaces Platinum from Mac OS 8 and 9.
It also adds the Dock as a place to launch applications (Not dissimilar to the underused Launcher) as well as some other features. It started out on PowerPC processors but later transitioned onto Intel processors with Mac OS X Leopard (10.5) the last to natively support the PowerPC architecture and Snow Leopard (10.6) to support PowerPC applications on Intel-based Macs. All versions of Mac OS X that were made to run on PowerPC systems (with the exception of Leopard) had a Mac OS 9 emulation layer called 'Classic'. It allowed Mac OS X to run Mac OS 9 applications that weren't updated to run natively on OS X (known as carbonization based on the Carbon API). However, some applications and extensions won't work under Classic. One example is Final Cut Pro 2.