MS-DOS - Almost the mother of all modern-day operating systems. Created by Microsoft Corporation it is a non-GUI system that relied heavily on commands to execute and run programs.  Still being used today and incorporated into all of Microsoft's systems and holds the foundations for most programming language.  Exp: C:\dir to read all the directories of the C drive.

Windows - A user friendly GUI that gives a graphic interface for the computer user to view, interact with, and manipulate. Eventually, it will bring television, telephone and computers closer together to work almost as one device for shopping, talking and workspace.
Exp: Windows 98, Windows 2000 and Windows XP.

Unix or Linux - Is the mother of all operating systems.  First developed by the United States Military along with German and Japanese scientists for military use.  With the help of MIT engineers and doctors of science it was first used to keep the science community in touch with one another.  Later to process coded and decoded information.  Now it is the basic foundation of all of if not most of the World Wide Web or the Internet and still the number one operating system in the world.

Macintosh's Systems - The currently operating system for Macintosh.