iMessage is an instant messaging service created by Apple. Its client program is called Messages, which is supported by their operating systems beginning with iOS 5 and macOS Mountain Lion.
iMessage allows users to send texts, documents, photos, videos, contact information, and group messages over Wi-Fi, 2G, 3G, or LTE. It provides an alternative to SMS/MMS messaging, allowing users potentially to save on text messaging fees.
The client program, Messages, shows an indication when the other user is typing. It also allows conversations to span across devices. A user can start a conversation on one device (such as the iPhone) and continue it later on another device (such as an iPad). All iMessages are encrypted and use the APNS (Apple Push Notification Service) protocol.
Apple terms, IOS, Messaging app, MMS, SMS, Text messaging
