Previewed at WWDC, launching in the fall.
Apple Seeds iOS 8 Golden Master to Developers Ahead of September 17 Public Launch
The update is available through Apple's over-the-air updating mechanism on iOS devices and it can also be downloaded via the iOS Developer Center. Apple has also seeded a golden master version of the new Apple TV software and Xcode 6.
Prior to the launch of the golden master, Apple seeded five iOS 8 betas to developers, with the fifth beta released on August 4.
iOS 8 introduces a range of new features, including improved integration with OS X through Continuity, a Health app, Family Sharing features, interactive notifications, a new QuickType Keyboard and improvements to several apps like Safari, Mail, and Messages, with all of the features being refined over the course of the beta testing period.