If you have a notebook, see this tip:
https://discussions.apple.com/docs/DOC-6413
PowerMac was the name given all Macintosh desktop (as opposed to notebook) computers with the IBM/Motorola PowerPC CPU. Their maximum operating system is Mac OS X 10.5.8 (if they have a minimum of 867 Mhz and a G4 processor on a single chip), and they can up to Mac OS X 10.4.11 use Classic, and some may be able to dual-boot Mac OS 9 and Mac OS X 10.5.8. For the purposes of posting in these forums, these were desktop Macs that did not have an attached display, and were not named Mac Mini. If you go to Apple menu -> About This Mac, and you find you have:
an Intel Xeon CPU, you have a Mac Pro. Mac Pros look like 
The latest Mac Pros can run the latest Mac operating systems.
Motorola and IBM made the CPUs 601, 603, 603e, 604, G3, G4, and G5 which were all PowerPC CPU (PPC) types. Powermac G5s looked most like the Mac Pro that was the desktop Mac from August 7, 2006 to October 21 2013, and only had one distinctive outside visual difference, and that's the fact they only had one optical drive bay:
All of these other machines were also PowerMacs:
Some 601 through 604 come from the age of Macintosh clones. The forum to post for PowerMacs is here:
If you have an display above a white dome on a silver articulating arm, you have an iMac G4, a type of iMac PPC.
iMacs may be distinguished using this tip:
Is my iMac an iMac PPC or Intel?
If you can't turn on the Mac, use https://discussions.apple.com/docs/DOC-6413 to identify your Mac further.
Much like the iMac, some Mac Minis were G4s, while others Intel.
This tip explains more about them:
Notebook Macs were labelled what they were either on the display frame, or their bottom side.
If your CPU was an 68000, 68020, 68030, 680LC40, or 68040 processor it is neither PowerPC, nor Intel, but considered Older Hardware by this forum's standards.
For more on software upgrade options, see this tip: