IOS utilizes a multi-touch interface in which simple gestures are used to operate the device, such as swiping your finger across the screen to move to the next page or pinching your fingers to zoom out. There are over 2 million IOS apps available for download in the Apple App Store, the most popular app store of any mobile device.
An operating system is what lies between you and the physical device. It interprets the commands of software applications (apps), and it gives those apps access to features of the device, such as the multi-touch screen or the storage.
Mobile operating systems like IOS differ from most other operating systems because they put each app in its own protective shell, which keeps other apps from tampering with them. This makes it impossible for a virus to infect apps on a mobile operating system, although other forms of malware exist. The protective shell around apps also poses limitations because it keeps apps from directly communicating with one another. IOS gets around this by using extensibility, a feature that lets an app be approved to communicate with another app.