Microsoft Windows, Palm-OS, Mac OS X, Ubuntu Linux, Google Chrome OS… whichever makes your computer/device/smartphone be able to run software such as calendars, email programs, telephone, contacts lists or Web browsers is an OS.
The newer it is and the more bits it operates on, the better. For example, Windows 7 (version is 6.1, not 7.0!) is better than Windows Vista (6.0) and even more than Windows XP (5.1 or 5.2).
And Windows 7 64-bit is -obviously- better than Windows 7 32-bit.
An OS is in reality a program. One that functions as a container of smaller ones, a huge package that allows other programs to be installed and work on top of it.
For example, Notepad is a program embedded in Windows, while Adobe’s Acrobat Reader can be installed and used on it.
Easy! Isn’t it?