What's a new iPhone without a new operating system? That's right, iOS 8 has been formally unveiled with the new iPhones, with a bunch of new features.
Starting with the new messaging app, which allows you to send small voice notes - similar to Facebook Messenger. There's also a new health app, and improved notifications. Extensibility also allows applications to talk to each other in real ways. We will have more on iOS 8 soon.
iOS 8 will be made to work with plenty of Apple devices, including the iPhone 4S, iPhone 5/5C/5S, most of the iPads, and the recent iPod.