iCloud is a system that allows users to backup, store, and share data such as text, photos, music and documents across multiple Apple devices for easy access from any device at any time. It works by automatically syncing files between a user's Mac, iPhone, iPad and other iOS devices either through a wireless internet connection or by transferring files manually through a web browser. iCloud replaces the need to sync directly with iTunes and allows content to be wirelessly accessed on any device linked to a user's iCloud account.