Now the iCloud gonna be your new digital hub (for the cloud haters: you can stay with your pc/mac as hub) and takes care of syncing and pushing your content to all your devices. Also iCloud can be integrated in any application (a new api:3) making it possible to sync data of all applications that support it.
Change your calendar, contacts, photos on one device and the data gets updated on all your other devices automatically.
It's fully integrated in the AppStore, iBooks and iTunes,
buy once and use it on all your other devices (no extra costs)
5gb storage for mail, documents and file backups.
Unlimited for apps,books and music (and the 30 day photo stream).
iCloud also provides a free @me.com mail adress.
All for free!
iCloud iOS features
Documents in the cloud
Basically a cloud based document control system for pages, keynote and numbers.
Make a photo on one device, it gets pushed to all your devices.
On OS X it syncs with iPhoto.
Integrated in the photo app.
Available on Apple TV.
All Photos get's back-upped online for 30 days.
iTunes in the cloud
No streaming as expected "just" auto sync for purchased music
...and one more thing
Music you ripped and which is available in the iTunes store gets automatically the same treatment as bought iTunes music including online backup.
You get a all your music replaced in 256kbps AAC (DRM free).
iTunes Match is the only of the services which is not available for free but with just 25$ a year its not very expensive.