iMessage is a free service from Apple that enables users of iOS-powered devices to send text messages over Wi-Fi from mobile devices like the iPhone and iPod touch. Prior to iMessage, most users needed to use a text message plan from a mobile provider in order to be able to send text messages from their iOS-powered device.
When sending text messages via iMessage, the mobile device's Messages app will first identify the recipient of the message prior to sending, and automatically send the messages via iMessage if the recipient's device uses iOS 5 (or later) or via the sender's regular text messaging service if the recipient doesn't have an iOS 5-powered device.
Users can easily distinguish messages sent via iMessage, as message replies appear in a blue-shaded bubble as opposed to the Messages app’s standard green-colored bubble.
Users of iMessage-capable devices can send an unlimited number of messages for free via iMessage. In addition to sending text messages, iMessage also supports sending photos, videos, Web links, contacts and location information. iMessage can also be used to send group messages to multiple people.