iCloud is Apple's cloud storage and computing service that allows users to store and access music, photos, files, and other data across devices. It launched in 2011, building on previous Apple cloud services. iCloud automatically backs up and syncs data between devices, finds lost devices on a map, and provides access to purchased media and apps on all devices linked to an Apple ID. It offers 5GB of free storage that can be expanded. While convenient for syncing across Apple devices, iCloud has limited compatibility and potential security issues when linked data is accessed from multiple devices.