Apple iOS is a proprietary mobile operating system that runs on the iPhone, iPad and iPod Touch. Apple iOS is based on the Mac OS X operating system for desktop and laptop computers. The iOS developer kit provides tools that allow for iOS app development.
Most modern iOS apps are written in the Swift language which is developed and maintained by Apple. Objective-C is another popular language that is often found in older iOS apps. Although Swift and Objective-C are the most popular languages, iOS apps can be written in other languages as well.