iOS is a mobile operating system developed by Apple. It was originally named the iPhone OS, but was renamed to the iOS in June, 2009. The iOS currently runs on the iPhone, iPod touch, and iPad.
Like modern desktop operating systems, iOS uses a graphical user interface, or GUI. However, since it is a mobile operating system, iOS is designed around touchscreen input, rather than a keyboard and mouse. For example, applications, or "apps," can be opened by a single tap, rather than a double-click. Different screens can be viewed by swiping your finger across the screen, rather than clicking on open windows.
Since iOS is designed to be simple and easy to use, it does not include several features found in a traditional operating system. For example, you cannot manage files and folders like you can in Mac OS X or Windows. You also have limited access to iOS system settings. Instead of modifying application preferences from within each program, most settings need to be adjusted within the Settings app. Additionally, while you can run multiple programs at once, you can only view one open program at a time.