iCloud is Apple's cloud storage and computing service that allows users to store and access data such as music, photos, documents, and app data across their Apple devices. It launched in 2011, building on Apple's previous cloud services. iCloud offers 5GB of free storage that can be upgraded, and syncs purchases, files, backups and other data wirelessly between devices. It also includes features like Photo Stream, Find My iPhone, and iCloud Keychain for storing passwords and credit cards securely online.