"IOS 7 is the biggest change to iOS since the introduction of iPhone," Cook said, opening Apple's annual Worldwide Developers Conference in San Francisco today (3am, Sydney time).
Icons, buttons and colour schemes have all been revamped.
Cook also previewed a new version of the Mac OS, dubbed Mavericks, that will include a new Maps application; and - as widely tipped - announced iRadio, a free ad-supported music streaming service, initially only for US users.
Apple exec Phil Schiller gave a tantalising glimpse of a wildly different Mac Pro - Apple's most powerful Mac, aimed at professional users - that resembles an upright cylinder, only one eighth the size of the current Pro machines. Due later this year, it will be powered by Intel Xeon chips with up to 12 cores.
In the new iOS 7, Apple's voice "assistant" Siri, has had a sex change. Siri now comes with a choice of male or female voices, both clearer than before.
Apple's iRadio service - due in the US autumn - will offer 200 "stations", some curated by Apple, others personalised by the user. Siri will be on iRadio too, answering commands like "Who plays that song?" or "Play more like this". Users can pay US$24.99 a year for an ad-free version.
Also announced: new versions of the MacBook Air lightweight laptop, with the latest Intel Haswell chips, said to give the Air up to 12 hours of battery life. They're available immediately in Australia at starting prices of A$1099 for an 11-inch model and $1249 for a 13-inch version.