iOS 9, codenamed "Monarch", is Apple's current-generation mobile operating system. It was announced at WWDC 2015 at Moscone Center in San Francisco on June 8, 2015. It includes a more intelligent Spotlight, more proactive Siri, a new News app, an updated Notes app, transit directions in Maps, a renamed Wallet app with store and loyalty cards, tether-less CarPlay, and slide-over and split-screen modes for iPad.
The gold master was released on September 9, 2015 during the iPhone 6s event, and the official release date was set for September 16, 2015. At the same time, iOS 9.1 "Boulder" was released in beta. iOS 9 is available now as a free download for supported devices.
iOS 9 supports the same hardware as iOS 8, so chances are if you're running iOS 8, you'll be good to go with iOS 9. Apple has made iOS 9 much smaller than iOS 8, so users won't run into issues with not having enough space to install the new OS. iOS 9 doesn't feature a drastic redesign from iOS 8, yet continues to build on apps and features. Other top new features of iOS 9 include improved Siri suggestions, Selfie and Screenshot albums in Photos, faster access to Apple Pay, low power mode, content blocking in Safari, new features in Notes app, quicker multitasking, and much more.
iOS 9.3, released on March 21, 2016, brought along a slew of improvements and new features, including Night Shift, touch ID support for Notes, new CarPlay features and much more. Read all about it right here.