iOS is the operating system used by Apple touch devices, including the iPhone and iPod Touch. Until 2017, it was also used by the iPad, which now uses iPadOS - a variant of iOS. Other devices like the Apple Watch use watchOS, a different but very similar operating system.
The operating system is upgraded every year, resulting in enhanced performance, bug fixes and changes to the user interface.
Users can access the App Store, a platform from which they can download all kinds of applications.