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Cell phone owners have long enjoyed (and abused in quantity) the ability to send and receive SMS texts, which turn out to be enormously expensive relative to the amount of data that’s actually being sent. With its Messages app, Apple broke the cellular carriers’ chokehold on texts by sending the same data over the Internet for free. The iPad isn’t a cell phone, but using the Messages app, you can communicate with other people who own a device running iOS 5.0 or later.
Instead of identifying you using a cell phone number, Messages uses your email address to route incoming messages from others. Messages uses your Apple ID by default. However, you can also specify other addresses that people can use to send messages to you.