Bootcamp is an official piece of software from Apple, that is built into the operating system of every Mac that's running OS X v10.5 or later. It is specifically made so that you can run Windows and its software on your Mac.
It's the best one by far, because other emulation software runs less efficiently, which means your Windows programs and games will run slower on your mac than they would on a exactly the same hardware specification PC. Due to the emulation process taking up your processor time.
However Apple how now eliminated this problem completly so that your Windows software will now run at full speed relative to the hardware inside the Mac, partly helped by the fact that Macs now use the same Intel processors as Windows PC's
You would need to get your self a copy of whichever Windows you want to use though. Look on ebay you can pick up copys for pretty cheap.
OS is the mac software
OS stands for Operating system. If its from the Shipsim requirements then its referring to 64 or 32 bit Windows 0S.
OSX is the Mac OS