Apple unveiled a new operating system Monday to make its computers act more like its mobile phones, while it also introduced a free "cloud" service that will wirelessly synch music, pictures and other files among a user's devices.

Ailing CEO Steve Jobs was on hand for Apple's big announcements, made at the company's annual developers' conference.

Jobs' public appearance was only his second since taking an indefinite medical leave of absence in January. Looking thin in his trademark jeans and black mock turtleneck, he alternated with company executives to announce Apple's latest innovations at its annual developers' conference.

Jobs emerged on stage to the sounds of the James Brown classic "I Feel Good," and was met with a standing ovation and a shout of "We love you" from the crowd.

Among its announcements Monday, the company unveiled a new service called iCloud, which stores a user's content -- from contacts and calendar events to music and photos -- and pushes it wirelessly to all of the user's devices. Anytime there is a change on one device, such as a new contact or an amendment to a document, the change is immediately updated on all other devices.

"Everything happens automatically and there's nothing new to learn," Jobs said of iCloud. "It just all works."

The service will be free to start, replacing the company's MobileMe service, which cost $99 and Jobs described as "not our finest hour."

The service also allows users to store their music online. A song purchased on iTunes, for instance, can be stored on up to 10 separate devices. For $24.99 per year, a special iTunes service called iTunes Match will scan a computer's hard drive for musical files that have been converted from CDs and other sources and upload them to iCloud.

The iCloud service also backs up data on Apple's servers.

Before unveiling iCloud, the Apple team unveiled a new operating system for Mac computers called Lion, which the company boasts contains more than 250 new features. The operating system aims to bring the touch-screen technology that is so popular with Apple's iPhones and iPads to its computers. With the swipe, tap or pinch of a finger, users will be able to zoom in or out, turn a page or move between apps, many of which will be available in full-screen mode.

Lion will be available in July as a download from the Mac App Store. It will cost $29.99.

ICloud will launch in the fall to correspond with Apple's updated software for iPhones and iPads, iOS 5. Users with an iPhone, iPad or iPod touch running iOS 5 or a Mac running OS X Lion can sign up for iCloud for free.

In describing the new operating system, the company said it also improves on the notification system for new messages or missed calls, and better integrates with Twitter.

In another of its announcements Monday, the company said it has sold more than 25 million iPads since the product first went on sale 14 months ago. Jobs' last public appearance was to announce the iPad's second incarnation in March.

On Apple's big day, the company's stock was down $4.05, or 1.2 per cent, to sit at $339.33 in afternoon trading.

With files from The Associated Press