Apple's text messaging system for iOS devices and Macs (OS X Version 10.8 and higher). Introduced with iOS 5 in 2011, iMessage enables users to send text, photos, etc. (the SMS and MMS services) to compliant devices without using the cellphone carrier's text messaging system.
Apple integrated regular text messaging and the iMessage service into the same app on the iPhone. If the recipient of a message is an Apple user, the Send button switches from green to blue. In addition, users can send short voice clips to iMessage recipients. See iOS versions