iOS is Apple's mobile operating system, second only to Android in market share. Key iOS applications include FaceTime for video calls between Apple devices, AirPlay for streaming media, Find My iPhone for device tracking, and iCloud for data backup. The App Store allows downloading third-party apps, and iOS is developed using Apple programming languages like Objective-C and Swift. Major iOS versions have introduced features such as the App Store, push notifications, multitasking, and Siri voice assistant.