The iOS is the operating system that runs the iPhone, iPod touch, and iPad. It's the core software that comes loaded on all devices to allow them to run and support other apps. The iOS is to the iPhone what Windows is to PCs or Mac OS X is to Macs. This article is a history of each version of the iOS, when it was released, and what it added to the platform.

iPhone Firmware History

iOS 9.0
Released: Fall 2015
Added:

• Maps app with mass transit directions
• Proactive, Apple's Google Now competitor
• Improved Siri
• Low power mode for up to 3 hours of extra battery life
• Overhauled Notes app
• New apps: News, Switch to iOS, iCloud Drive
• Improvements to HomeKit and CarPlay
• ?Smaller file size for apps, less free space needed to update OS
• Numerous other improvements
• Support for iOS Public Beta program

iOS 8.4
Released: June 30, 2015
Added:

• Apple Music
• Fix for text-message bug that could crash phones with a single message
• Other improvements and bug fixes

iOS 8.3
Released: April 8, 2015
Added:

• Support for multicultural emojis
• Siri support for Russian, Danish, Dutch, Thai, Swedish, Turkish and Portuguese
• Ability to report iMessages as a spam
• Numerous more subtle improvements and bug fixes 

iOS 8.2
Released: March 9, 2015
Added:

• Support for Apple Watch
• Apple Watch app that provides configuration options and App Store access
• Improvements to the Health app
• and many other improvements and bug fixes.

iOS 8.1
Released: Oct. 20, 2014
Added:

• Support for Apple Pay

• The return of Camera Roll to the Photos app (it had been removed in iOS 8.0)
• iCloud Photo Library beta
• Support for iPad Air 2 and iPad mini 3
• and many other improvements and bug fixes.

iOS 8.0

Released: Sept. 17, 2014
Added:

• Notification Center widgets
• Third-party keyboards
• App extensions to provide functionality between apps
• Support for Apple's Continuity and Handoff features with Mac OS X
• iOS-to-Mac AirDrop support 
• Healthkit support
• Homekit support
• Improvements to the Messages app, including voice messages and auto-expiring messages
• Family Sharing
• Wi-Fi calling.

iOS 7.1

Released: March 10, 2014
Added:

• Support for CarPlay
• New voices for Siri in Mandarin Chinese, UK English, Australian English, and Japanese
• Added iTunes Radio features
• Improvements to the Touch ID fingerprint scanner on the iPhone 5S
• Enhancements to the Calendar app and accessibility features
• Changes to improve performance on the iPhone 4
• and many other improvements and bug fixes.

iOS 7.0.6

Released: Feb. 26, 2014
Added:

• Bug fixes for major iOS security hole that allowed websites to fake security certificates

iOS 7.0.5

Released: Jan. 29, 2014
Added:

• Bug fixes for iPhone 5S and 5C running on Chinese wireless networks

iOS 7.0.4

Released: Nov. 14, 2013
Added:

• Mostly unspecified bug fixes

iOS 7.0.3.
Released: Oct. 22, 2013
Added:

• New controls to reduce OS-level motion and animations
• Bug fixes
• iCloud Keychain
• Password Generator

iOS 7.0.2
Released: Sept. 26, 2013
Added:

• Fix for lockscreen bug
• Greek language support to lockscreen

iOS 7
Released: Sept. 18, 2013
Added:

• New user interface
• iTunes Radio
• iTunes in the Car
• Control Center
• Improvements to Find My iPhone, including Activation Lock
• Touch ID fingerprint scanner on iPhone 5S
• AirDrop
• Image filters built in Camera app
• Hundreds of smaller changes

iOS 6.1.5
Released: Nov. 14, 2013
Added:

• Bug fix for 4th generation iPod touch

iOS 6.1.4
Released: May 2, 2013
Added:

• Bug fix for iPhone 5

iOS 6.1.3
Released: March 19, 2013
Added:

• Bug fixes

iOS 6.1.2
Released: Feb. 19, 2013
Added:

• Bug fixes for Microsoft Exchange

iOS 6.1.1
Released: Feb. 6, 2013
Added:

• Bug fixes for iPhone 4S

iOS 6.1
Released: Jan. 28, 2013
Added:

• Improvements to Siri
• More carrier support for LTE networks
• Modifications to iCloud at device set up
• Numerous minor enhancements
• Bug fixes

iOS 6.0.2
Released: Dec. 18, 2012
Added:

• Bug fixes on iPhone 5 and iPad mini

iOS 6.0.1
Released: Nov. 1, 2012
Added:

• Bug fixes

iOS 6
Released: Sept. 19, 2012
Added:

• Replaced earlier Maps app with new, Apple-created Maps app
• Turn-by-turn directions
• Do Not Disturb
• Passbook
• New privacy controls
• Removed YouTube app
• Expanded features for Siri
• Facebook integration
• Support for FaceTime over cellular networks, where available
• Bug fixes