iCloud is Apple's personal cloud service for storing and syncing photos, files, notes, settings and backups across devices. It is built into iOS, iPadOS, macOS and other Apple operating systems, is available for Windows and has a limited web interface at iCloud.com. Components include iCloud Drive, iCloud Photos, backups, email, passwords through Keychain and Find My. There is a free storage tier and a paid tier, iCloud+.
Apple is a US company, so US legislation such as the CLOUD Act can apply. According to Apple, Apple and third-party data centres may store and process data, and the keys are always held in Apple data centres. By default 15 categories are end-to-end encrypted; with the optional Advanced Data Protection that rises to 25. Mail, Contacts and Calendars are not end-to-end encrypted. iCloud is not open source and cannot be self-hosted.