iOS is the operating system Apple has developed for its mobile and tablet devices (iPhones and iPads). iOS is the underlying operating system that controls the hardware and interfaces for the iPhone and iPad.
Apple provides developers with an iOS SDK (Software Developer Kit) which allows others to create and distribute Apps with all the functionality the operating system provides, including using the hardware such as the camera, GPS, accelerometer, etc.
iOS would be the equivalent to Windows or Linux or macOS for desktop computers.
