An operating system is software that allows communication between the hardware (e.g. hard drive) and programs such as Microsoft Word.
Some examples of operating systems are Microsoft Windows 2000, Microsoft Windows XP, Mac OS X, Red Hat Linux, and many others.
How do you use it?
One uses an operating system by installing it on his or her computer. After it has been installed other programs can be installed such as Microsoft Word and Firefox. Thereby, allowing one to type documents and surf the Internet.