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iCloud is a cloud storage and cloud computing service from Apple Inc. launched on October 12 2011. As of July 2012, the service has more than 150 million users.
The service allows users to store data such as music and iOS applications on remote computer servers for download to multiple devices such as iOS-based devices running iOS 5 or later, and personal computers running OS X 10.7.2 "Lion" or later, or Microsoft Windows (Windows Vista service pack 2 or later). It also replaces Apple's MobileMe service, acting as a data syncing center for email, contacts, calendars, bookmarks, notes, reminders (to-do lists), iWork documents, and other data. The service also allows users to wirelessly back-up their iOS devices to iCloud instead of manually doing so using iTunes.

Read more >http://en.wikipedia.org/wiki/ICloud