If possible, go to Apple menu -> About This Mac to know what you are working with. If not use one of the ways of visually identifying your iMac described below.
PPC = PowerPC, referring to the processor found in Macs before 2006 known as 601, 603, 604, G3, G4, and G5.
These were made by Motorola and IBM to varying degrees. In spite of that fact, they are less compatible with IBM PC compatibles
than the newer Intel Macs.
iOS which runs on iPhones, iPod Touch, iPad requires an Intel Mac to sync with iOS 6 or later. For more on that, see this tip:
Intel is not PPC, and PPC is not Intel.
If you have 10.6 or later Mac OS X installed, you don't have a PPC.
If you have Boot camp installed, you don't have PPC.
If you have 10.4.4 (including 10.4.10 and 10.4.11, but not 10.4.1) through 10.5.8 installed, you may have PPC, or you may not
as those were the only Mac OS X versions written for both CPU types.
If you have Mac OS 8.1 through 9.2.2 (without X), you have PPC.
If you have Mac OS X 10.0 through 10.3.9 you have PPC.
If you have Mac OS X 10.4.0 (the zero is not mentioned after the 4.), 10.4.1 (not to be confused for 10.4.10 or 10.4.11), 10.4.2, and 10.4.3 you have PPC
Mac OS X 10.4.3 and earlier could only be installed on PPC Macs.
As a side note, to ease the migration from PowerPC to Intel, Apple offered an environment known as Rosetta in 10.4.4 through 10.6.8 allowing applications
developed for PowerPC to be used on Intel Macs. This environment is still available on newer operating systems with a virtualizaiton solution.
Here's how to identify iMacs visually. There are three main series of iMac PPC (PowerPC), iMac G3, iMac G4, and iMac G5:
During the middle of the iMac G3 series, Apple allowed just a plain Airport card adding, whereas earlier it required a special adapter. Alas, all iMac G3s only support the 802.11b airport card, and that does not support secure wireless connections with WPA2. For those you'll need an ethernet/wireless 802.11g adapter. Airport compatibility is here:
iMac G3s with Firewire ports can also be upgraded to 10.4.11. Those without Firewire could only be updated to 10.3.9.
See this image to determine if they have Firewire ports:
Note, USB 2 was added and 802.11g Airport Extreme card support was added during the middle of the iMac G4 series.
802.11g Airport Extreme compatibility is identified here:
The 1 Ghz and faster models could be upgraded to Mac OS X 10.5.8, but could not use Classic on 10.5 or later.
The 800 Mhz Superdrive model and 700 Mhz model both supported Mac OS 9 booting. The 800 Mhz combo drive model
only supported Classic for Mac OS 9. To understand the difference, see this tip:
This one may be PPC, or Intel depending on the EMC# read off the bottom of the foot:
Others are PPC and known as iMac G5.
The G5 supported Classic. And when upgraded past 10.4.11 could not support Classic on the same boot. It could though run up to Mac OS X 10.5.8. That does not support syncing with iOS 6 or later, as this tip explains:
The Intel supports Boot Camp on Mac OS X 10.5 and later, but does not support any Classic.
If you definitely have an iMac PPC, please post here:
This model as it appears below is clearly Intel. That aluminum front bezel on an LED or LCD is only used on Intel iMacs.
If you definitely have an iMac Intel, please post here: