iCloud is Apple's cloud storage service that securely stores users' photos, files, passwords and other data across devices. It allows seamless integration between Apple devices so that any changes made on one device are instantly reflected on others. iCloud provides 5GB of free storage but users can subscribe to iCloud+ for more storage and additional features like privacy tools. iCloud protects data through encryption both in transit and at rest, and uses techniques like two-factor authentication and end-to-end encryption to ensure only authorized devices can access users' information.