You can use the Messages app on your iPhone, iPad or iPod touch to send messages. Those messages will be sent as iMessage or SMS/MMS. Find out more about the difference between the message types.
iMessage
iMessages are texts, photos or videos you can send to another iPhone, iPad, iPod touch or Mac over Wi-Fi or mobile 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're not using iMessage, you can use SMS/MMS. These messages are texts and photos you can send to other mobile phones or another iPhone, iPad or iPod touch. SMS/MMS messages won't be encrypted and will appear in green text bubbles on your device.
To use SMS/MMS on iPhone, you'll need a text messaging plan. Contact your wireless network provider 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 mobile data. Mobile data rates may apply.