PPC indicates a Mac using the PowerPC chip. These include all Macs that were new from 1995 to 2005.
Apple's iMac lineup that is PowerPC is identified on this article: http://support.apple.com/kb/HT1463
If the issue you are having does not appear to change with operating system updates, and is system wide, please post to this forum, if the iMac is G3, G4, or G5: https://discussions.apple.com/community/desktop_computers/imac_powerpc
If you have an Intel iMac, identifying them is taken care of by this article: http://support.apple.com/kb/HT1758
If the issue you are having does not appear to change with operating system updates, and is system wide, please post to this forum, if the iMac is Intel: https://discussions.apple.com/community/desktop_computers/imac_intel
All Macs that were announced new starting in 2006, had the Intel chip, though some leftover PowerPC Macs were sold in 2006. Be sure to identify the Mac carefully in the article before choosing which forum to post in.
Operating system notes:
1. iMac Intels could run their prebundled DVDs and newer retail operating systems. and no Classic, or Mac OS 9 support.
CoreDuo iMacs can not run Mac OS X Lion 10.7 or later. And all Intel Macs with less than 1 GB of RAM could not run 10.6 or later.
2. iMac PowerPCs could only run up to Mac OS X 10.5.8 if they had a G5, or a G4 processor with 1 Ghz or higher and a Firewire or internal DVD drive. No
Boot Camp is available to them, and no Mac OS 9 is available to them if only 10.5 (Leopard) or later is installed. A third party program known as LeopardAssist could support some 800 Mhz iMacs.
3. iMac PowerPCs with Firewire an a Firewire DVD drive or built-in DVD drive could run 10.4 (Tiger) Retail if they were released prior to April 29, 2005.
They had a serial number of xx417xxxxxx or less. 10.4.3 retail supported serial numbers of xx441xxxx or less of this description. X can be any letter or number.
And the numbers and letters following the described 3 numbers could be any number of letters or numbers. They also had to have the proper firmware installed from http://support.apple.com/kb/ht1395 before going to 10.2 or higher while booted from Mac OS 9. A limited release Media Exchange Program 10.4 on CD allowed those with only CD drives to use Tiger. 800 Mhz iMacs with combo drives (as opposed to Superdrives which had DVD burning capability the combo drives did not) and 1 Ghz and higher iMacs no longer supported Mac OS 9 booting, but still supported Classic up to 10.4.11.
4. These 10.3 Panther versions were supported on retail releases on iMacs that had the following serial numbers with the firmware link (in note 3) having been installed in the same manner:
10.3.5 (Panther) xx432xxxx or earlier
10.3.2 xx351xxxxx or earlier
10.3 xx343xxxxx or earlier.
5. These 10.2 Jaguar versions were supported on retail releases on iMacs that had the following serial numbers with the firmware link (in note 3) having:
10.2.3 xx251xxxx or earlier.
10.2 xx234xxxx or earlier.
10.2's release also marks the first date in which Mac OS 9 was no longer bundled as a separate disc and had to be restored from the prebundled installer discs that were labelled something other than the operating system itself.
6. xx134xxxx and earlier iMacs could run off the 9.2.1 retail installer.
10.1.5 and earlier are not mentioned as few Mac OS X titles available today were made compatible with it and earlier versions of Mac OS X.
Here's how to identify iMacs visually. These two are PPC (PowerPC):
This one may be PPC, or Intel depending on the EMC# read off the bottom of the foot:
EMC# 2104, 2105, 2110, 2114, 2118, 2111, 2133, or 2134 are all Intel. Others are PPC.
This one is Intel: