Snow Leopard is the last version of OS X to support Rosetta, which allowed you to run PPC-based older Mac apps on Intel machines.You cannot run Snow Leopard on a Power PC Mac -- 10.5 Leopard was the final release for those models.
We'll tackle the downgrade question a bit later in this guide.
Once you install the OS, you can use the Mac App Store to update to any of the later versions of OS X, as well as purchase and install many apps for OS X.
Let's take the upgrade or downgrade question as two separate inquiries.
The writing was on the wall for Snow Leopard in December 2013 when Apple patched Safari for OS X Lion, Mountain Lion, and Mavericks without thinking about their older sibling.
Snow Leopard had received a security update in September of 2013, just two months earlier.