iCloud is Apple's cloud storage and computing service that allows users to store and access music, photos, documents, and other files across their Apple devices. Key features of iCloud include storing and syncing purchased content, photos, files and backups across devices; finding lost Apple devices on a map; keeping passwords and settings updated across devices; and automatically backing up iOS devices daily over Wi-Fi. iCloud offers advantages like easy syncing of content between devices without cables, but also has some disadvantages like compatibility only with Apple devices and possible security issues when multiple devices are synced to one account.