With the Messages app, you can exchange messages with other SMS and MMS devices using your cellular connection on iPhone, and with other iOS devices and Mac computers using iMessage.
iMessage is an Apple service that sends messages over Wi-Fi or cellular connections to other iOS devices (with iOS 5 or later) and Mac computers (with OS X 10.8 or later). These messages don’t count against your messaging plan. Messages sent via iMessage can include photos, videos, and other info. You can see when other people are typing, and send read receipts to let them know when you’ve read their messages. For security, messages sent via iMessage are encrypted before they’re sent.
iMessage texts appear in blue bubbles, and SMS/MMS texts appear in green bubbles. For more information about SMS/MMS and iMessage, see the Apple Support article Use Messages with your iPhone, iPad, or iPod touch.
Sign in to iMessage. Go to Settings > Messages, then turn on iMessage.
If you’re signed in to iMessage with the same Apple ID on all your devices, all the messages that you send and receive on iPod touch can also appear on your other Apple devices. You can send a message from whichever device is closest to you. You can even start a conversation on one device and continue it on another with Handoff.
Sign in to iMessage with the same Apple ID on your other Apple devices.
iOS devices: Go to Settings > Messages, then turn on iMessage.
Mac: Open Messages. If you’re signing in for the first time, enter your Apple ID and password, then click Sign In. If you signed in before and want to use a different Apple ID, choose Messages > Preferences, click Accounts, then select iMessage in the Accounts list.
With Continuity, all the SMS/MMS messages that you send and receive on your iPhone can also appear on iPod touch. For more information, see the Apple Support article Use Continuity to connect your iPhone, iPad, iPod touch, and Mac.