Types of operating systems
Operating systems usually come preloaded on any computer you buy. Most people use the operating system that comes with their computer, but it is possible to upgrade or even change operating systems.
The three most common operating systems for personal computers are Microsoft Windows, Mac OS X, and Linux.
Modern operating systems use a Graphical User Interface, or GUI (pronounced "gooey"). A GUI lets you use your mouse to click on icons, buttons, and menus, and everything is clearly displayed on the screen using a combination of graphics and text.
Each operating system's GUI has a different look and feel, so if you switch to a different operating system it may seem unfamiliar at first. However, modern operating systems are designed to be easy to use, and most of the basic principles are the same.
In the images below, you can see the Windows and Mac OS X GUIs.
Before GUIs, computers had a command-line interface, which meant the user had to type every single command, and the computer would only display text.