imessage

About iMessage and SMS/MMS

Learn why some of your message bubbles are blue or green.

You can use the Messages app on your iPhone, iPad, or iPod touch to send messages. Those messages are sent as iMessage or SMS/MMS. Learn more about the difference between the message types.

iMessage

iMessages are texts, photos, or videos that you send to iOS devices and Macs over Wi-Fi or cellular-data networks. These messages are always encrypted and appear in blue text bubbles. To turn iMessage on or off, go to Settings > Messages.

SMS/MMS

If you aren’t using iMessage, you can use SMS/MMS. These messages are texts and photos that you send to other cell phones or iOS devices. SMS/MMS messages aren't encrypted and appear in green text bubbles on your device.

To use SMS/MMS on an iPhone, you need a text-messaging plan. Contact your wireless carrier for more information. You can also set up your other Apple devices to send and receive messages from any Apple device.

If Wi-Fi is unavailable, iMessages will be sent over cellular data. Cellular data rates might apply.

About iMessage and SMS/MMS - Apple Support

iMessage works over the data network between compatible iOS devices that have activated it. It is not limited to 160 characters and you get a message that it has been delivered and there is no charge for it.

http://www.apple.com/iphone/built-in-apps/messages.html

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