iMessage enables users to send free messages to other iPad, iPhone, iPod touch, or Mac users. To receive messages from others they must know the phone number or email address that is registered with iMessage.
To find this, open Settings, select Messages, and tap Send & Receive.
Once you and another user know each other’s number or email address, you can exchange texts via the Messages app. Your conversations will also appear on your other registered devices, so you can switch between your devices and carry on chatting.
To tell if you are sending an iMessage, the Send button will turn blue and all the messages you send will have a blue background.