If your iMac is ON, then it is probably working perfectly normally.
PC people get confused by the fact that the Mac doesn't require this level of "micro-management." They also particularly seem to think it is odd that the top of their iMac is rather warm. This is called convection cooling (aka "heat rises") and is used by Apple's super-clever engineers to reduce the need for excessive fan noise by allowing the machine to vent heat naturally. Cool air is drawn in from the bottom, passes over the components and warms up, with the heat venting near the top. It's ingenious, really.
Should your machine *actually* overheat (which almost never happens), it will automatically shut down to protect the processor. So if that's not happening, your machine is likely working exactly as designed. As HarryB points out above -- computers are perfectly okay with being hotter than humans would find comfortable.