Messaging, telephony, videophone and collaboration service for iOS and MacOS devices
Technical description of Apple Services' security measures is available at https://ssl.apple.com/iphone/business/docs/iOS_Security_Feb14.pdf
Messaging and video/audio is end-to-end encrypted, keys are exchanged via Apple's proprietary global device directory. Apple is technically able to insert additional third-party encryption keys of required by law, but it is not mentioned in the description whether Apple does so. The enrcyption model itself can be considered safe. Attached files and photos are encrypted and saved into iCloud. The key and address are sent as an iMessage to recipient(s).
Apple ID or a valid telephone SIM in an iPhone
Apple Inc., Ireland