During the keynote address at Monday’s Worldwide Developers Conference, Apple CEO Steve Jobs unveiled iCloud, Apple’s new wireless data sync service for iOS devices, Macs, and PCs.

“Some people think the cloud is just a hard disk in the sky,” Jobs told the attendees. “We think it’s way more than that. And we call it iCloud.”

The free iCloud takes the place of Apple’s MobileMe service, which Jobs admitted, “wasn’t our finest hour.” Like MobileMe, iCloud offers the ability to sync contacts, calendars, and mail across devices. Create a new contact on your iPhone, for example, and it gets automatically pushed to the cloud and syncs to all of your devices and computers.

The same goes with calendar events, and you can even share calendars directly from your iOS device.

iCloud syncing also work with Safari bookmarks and books in iBooks. And iCloud isn’t just for Apple’s software, either. Third-party applications can store documents in iCloud and auto-sync them when changes are made on any device.

Auto-backup

Another aspect of iCloud is automatic data backup. Once a day, iCloud will back up a lot of a user’s important content to the cloud over Wi-Fi. If you ever get a new phone, Apple said, type in your Apple ID and password and everything will be loaded on that phone automatically.

It will back up purchased music, apps, and books, your Camera Roll (photos and videos), device settings, and app data.

New apps

Apple has also created three new apps for iCloud.

Documents in the Cloud uploads Pages, Numbers, and Keynote documents to the cloud for syncing.

Photo Stream lets you take photos on any device and uploads them to sync with your other devices (so your iPhone shots appear on your iPad, say). Photo Stream also works with iPhoto on Mac, and supports the Apple TV as well. Because photos are so large, Apple will limit Photo Stream to your last 1000 photos on iOS devices (unlimited on Macs and PCs). They’ll remain on iCloud for 30 days after you upload them, but you can save them permanently on a device by move them to a new or existing album.

Finally comes iTunes in the Cloud. For songs you’ve already purchased from the iTunes Store, there’s a Purchased button that shows you your entire purchase history of songs (purchased on any device). You can view by all songs, recent songs, or by artist. You can then download any song or album with the touch of a button.

“This is the first time we’ve seen this in the music industry,” said Jobs. “No charge for multiple downloads on many devices.”

You can download music to up to 10 devices, and you can also enable an Automatic Downloads feature that will grab any new purchases you make on other devices.

Apple also announced that APIs for iCloud will be released to app developers. It's possible that non-Apple apps will be able to take advantage of iCloud features, such as Documents in the Cloud.

iTunes Match

But what about music that wasn’t purchased from the iTunes Store—for example, all those CDs you’ve ripped yourself? Apple has come up with iTunes Match, a $25-a-year service that scans your iTunes library and tries to match it with some of the 18 million songs that Apple sells. For the songs that are in the iTunes Store, you get instant access, and Apple will even upgrade them to its standard 256-kbps AAC format. For those songs that Apple can’t identify, you can upload them manually. Don't get too excited though - iTunes Match will be US only, with Apple giving no indication when or even if it would be available in the UK.

Specs and availability

Each user gets 5GB of free storage for mail, documents, and backup. But on the plus side, purchased music, apps, books, and Photo Stream photos don’t count against that total.

iCloud will ship at the same time as iOS 5 in the autumn, although developers can download a beta version now.