iCloud is Apple's cloud storage and computing service, launched in 2011, that allows users to store and access data like music, photos, and documents from any Apple device. It offers a variety of features like backup and restore, Find My iPhone, iCloud Keychain, and iWork apps. iCloud provides automatic syncing across devices and 5GB of free storage. While it offers convenience, some issues include limited compatibility, needing iTunes to be open to sync, and potential security risks when multiple devices are synced to one account.