iCloud is Apple's cloud storage and synchronization service that backs up photos, documents, contacts, calendars, and device settings across a user's Apple devices. It powers features like automatic iPhone backups, Desktop and Documents syncing on Mac, Photo Library sharing, and Find My device tracking, and lets files be accessed through a web browser at iCloud.com or through an official Windows app.
iCloud is built into every Apple device and is central to using an iPhone, iPad, or Mac, since an Apple ID and iCloud account are required for backups, app data sync, and Family Sharing. Every Apple ID includes 5 GB of free storage, while iCloud+ subscription tiers add significantly more space along with privacy features like Private Relay and Hide My Email.
The service is available on iOS, iPadOS, macOS, Windows, and the web, with no native Android or Linux client. Files download for offline access and re-sync automatically, and developers can build iCloud-backed data sync into their own apps through Apple's CloudKit framework.