UPDATE: You get access to iCloud when you upgrade or buy a new phone. Everyone gets 5GB of iCloud storage for Mail, but don't worry because Apps, Books and Photo Stream do not count. Developers can try out iCloud from today.
Steve Jobs has just unveiled iCloud and it is a service that frees all your devices from having to connect to your Mac to get updated media and data. The Mac is being demoted, and in its place is the iCloud which will become "the hub of your digital life". iCloud will be free!
We've got a solution for this problem. We're going to demote the PC and the mac to just be a device. We're going to move your hub, the center of your digital life, into the cloud."
iCloud stores all your content, so that if you have something new on your iPhone it is sent to the cloud immediately, iCloud will then push it back to all your other devices.
And it's completely integrated with your apps, and there's nothing new to learn. It just all... works. It just works.
Whether it be new events on your calendar or a new message in your @me account (which will remain), all of it gets pushed to the cloud and then pushed back to all your devices. The App Store, which is integrated within iCloud will let you see all your purchased apps, even if they are from another device. The same will occur for iBooks
Now what about for devices you buy in the future. The cloud downloads it to all of your devices.
Backup too will be dealt with by iCloud, it will take daily backups of all your devices over WiFi including all the purchased music, apps and books, photos, videos and app data. If you ever lose your device or get a new one, it can all be restored from iCloud.