iCloud is the cloud storage and sync service of Apple Inc. It keeps a user's photos, files, notes, contacts, calendars, passwords and messages backed up on Apple's servers and up to date on all of the user's devices. The data can also be opened in a browser at iCloud.com.
App developers use iCloud through CloudKit, a framework (a set of code libraries) and tools for storing an app's data in the user's iCloud. The app saves its data through CloudKit, and iCloud keeps it current on iOS, iPadOS, macOS, tvOS, watchOS and visionOS devices and on the web, while Apple handles scaling, consistency and security. The CloudKit Console gives developers usage telemetry, server logs without personal data, dashboards, and a tool to send and check push notifications.
Every user gets 5 GB of iCloud storage free. iCloud+ is a monthly subscription with more storage, from 50 GB to 12 TB, and privacy features such as Private Relay, which stops websites from profiling a Safari user by IP address and location, and Hide My Email, which creates random forwarding addresses. A plan can be shared with up to five family members.