iOS is a mobile operating system developed by Apple Inc. and distributed exclusively for Apple hardware. It is the operating system that powers iPhone, iPad and iPod Touch. Each main version of iOS / iPhone OS has introduced a big, main feature of the new version.

• iPhone OS 1 was the original "iOS", and had only first-party Apple, preinstalled apps such as "Phone", "iPod", and "Safari".
• iPhone OS 2 introduced the App Store that we know today.
• iPhone OS 3 added in copy and paste functionality, several Google-licensed apps (Maps and Youtube), as well as basic tethering functions.
• iOS 4 brought in multitasking and Wallpaper-changing. iOS 4 also dropped the original iPhone from it's supported devices, and the iPhone 3G held limited support.
• iOS 5 introduced several new features, such as iMessage, Newsstand, Reminders, Twitter integration, iCloud, and a beta of Siri.
• iOS 6 fully released Siri and Siri Eyes Free, brought in Passbook and Facebook integration, as well as Apple-sourced maps instead of Google sourced, marking the removal of the Youtube app as well.
• iOS 7 held a massive design overhaul, turning skeumorphic into modern and flat. Also brought in a "Control Center" which offered a quick-access area for commonly used features (flashlight, wifi on/off, bluetooth on/off, calculator app, etc.).
• iOS 8 introduced the Health App, which enables data-mixing from all kinds of fitness apps, from MyFitnessPal to Misfit. iMessages gained voice messaging, and Indoor Mapping with the iPhone 5S, 6, and 6+'s M7 and M8 coprocessors.
• iOS 9 introduced a new multitasking features on iPad, new Notes features, the iCloud Drive app, HomeKit integration, Apple Maps improvements.
• iOS 10 incorporated changes to 3D Touch and the lock screen. There were new features to Messages, Maps, Photos and Siri.
• iOS 11 introduces significant design changes: combined lock screen and Notification Center, Appstore restyle, translation between languages for Siri, screen recording.