What Is iCloud?
iCloud is a new service coming this fall that will allow you to easily and automatically synchronize all your data between various Apple devices. Music, documents, calendars, contacts, mail, photos, apps, etc. will be pushed to all your devices on the fly. If you buy a song in iTunes on your Mac, you'll find it on your iPad next time you use it.
The first 5GB of storage space will be free with iCloud. However, your music, photos, books, and apps will not count toward the 5GB!