CloudKit is an integrated macOS, iOS, iPadOS, watchOS, and tvOS API that functions as a backend as a service (BaaS).[1] CloudKit is the framework that powers iCloud on those operating systems and on the web.
Services
Application developers can utilize CloudKit for integrated access to Apple's iCloud servers into iOS and macOS applications. The framework provides authentication, a private database, a public database and structured asset storage services allowing developers to focus on client-side development.[2] It is the foundation for both iCloud Storage and iCloud Photo Library.[3]
CloudKit also offers several APIs to access the iCloud Storage, where a user can store data and files so that they can be easily accessible from other devices.[4]