iOS is a mobile operating system developed by Apple. It currently runs on the iPhone. It previously ran on the now-discontinued iPod Touch; a version also ran on the iPad, which now runs a separate fork called iPadOS.
Since the iPhone is a touchscreen device, the iOS graphical user interface utilizes touch and voice input instead of a keyboard and mouse. You can tap, hold, or swipe your finger on the touchscreen to interact with apps and objects on the screen. Tapping and swiping on certain parts of the screen lets you move between apps and access special system screens. For example, tapping on an app's icon on the Home screen launches that app, swiping down from the top shows the Notification center, and swiping left and right moves between Home screens. You can also use the Siri voice assistant to issue some commands and use speech recognition in the place of keyboard input.
Since iOS is designed for mobile devices, it provides a simple user experience and lacks some features found in desktop operating systems. For example, iOS applications must run sandboxed, limiting their access to the hardware and other apps. The iPhone screen only displays the interface for one app at a time; while apps can perform tasks in the background, you cannot interact with more than one at a time. Your access to the file system is also limited to certain folders. However, new features are introduced in every annual update as the operating system matures. iOS includes a suite of first-party software, like productivity tools, messaging apps, information apps, and a web browser. Third-party apps for iOS are exclusively available in its App Store.