How Does iMessage Work
iMessage is an Apple app that lets you send text messages to other iOS devices only via your data connection over the Internet instead of your standard text messaging provider. If one already has a data plan, iMessage cost nothing.
If your iOS device attempts to send an iMessage, but the person you are texting does not have an iOS device, your iOS device will send the message via standard text messaging services. iMessaging may save you money depending on your data plan.
To use iMessage, simply turn it on in your device settings. If your other iOS devices are synced, all your synced devices will receive the iMessage. If you use the same email address for all your devices or the same Apple ID, that is. If someone iMessage your phone number, your other devices will not receive that text message.