OS X is the operating system for macs. Windows is the operating system of PC's. OS X is made by Apple, and so it your iMac. Easy way to put it;
iMac is the hardware you bought from apple.
OS X is the software that runs on your iMac that is made by apple.
Running OS X (which comes pre-installed) is running Mac. Putting Windows on it makes it run as a PC.
Bootcamp is a program built in where you can easily install Windows on a Mac. You would do this if you need Windows, but never wanted to buy another computer to run Windows. It allows you to jump between Mac (OS X) and Windows with just a restart.
If your Steam games show they work with Mac (OS X), then you won't need Windows. Some games use Windows but I feel you won't really need to install it for a what you are wanting to play.
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Well I would say that someone (if not you) should run it and use it for a week. Even if they just switch it on and make sure basic things work (sound, web cam, screen, keyboard, mouse, check for scratches...).