iMessage is an Apple service that lets you send unlimited messages over Wi-Fi (or cellular connections) to others using iOS 5 or later, or OS X Mountain Lion or later. With iMessage, you can see when other people are typing, and let them know when you've read their messages. iMessages are displayed on all of your iOS devices logged in to the same account, so you can start a conversation on one of your devices and continue it on another device. For security, iMessages are encrypted before they're sent.