Not sure what OS stands for? You hear it mentioned all the time, but you're just not 100 percent positive of what it means. That's okay, no problem. OS is short for Operating System. Windows is the most common operating system, but Linux and Mac OS X are also choices.
Whatever flavor you enjoy, they all do pretty much the same thing. They give you a way to run programs and work with your computer. Whether it be Windows 95, 98, ME, XP, etc., they all basically just let you use your computer, because it surely wouldn't work without one.
So, next time someone is talking about their OS, you'll be able to give the nod of understanding that only comes from knowing your acronyms.