iOS is Apple's mobile operating system. An iPhone uses iOS as its operating system.
Like many modern operating systems, iOS has a graphical user interface. Due to its mobile nature, iOS has been designed for touchscreen input rather than a mouse and keyboard. The opening of applications, or "apps," can be performed by simply tapping rather than clicking twice. Instead of clicking on open windows, you can swipe your finger across the screen to view various screens.
Because iOS is designed to be simple and easy to use, it does not include several features that are common in traditional operating systems. Files and folders cannot be managed like in Windows or Mac OS X. There is also limited access to iOS settings. In most cases, settings can be adjusted within the Settings app instead of within individual programs. Moreover, one can only open one program at a time, even though multiple programs can be run at the same time.