Messages lets you exchange text messages with other SMS and MMS devices using your cellular connection, and with other iOS devices and Mac computers using iMessage.

iMessage is an Apple feature that lets you send messages over Wi-Fi (or cellular connections) to others using iOS 5 or later, or OS X Mountain Lion or later. Messages you send using iMessage don’t count against your text messaging plan.. Messages can include photos, videos, and other info. You can see when other people are typing, and let them know when you’ve read their messages. If you’re signed in to iMessage using the same Apple ID on other iOS devices or a Mac (OS X Mavericks or later), you can start a conversation on one device and continue it on another. For security, messages you send with iMessage are encrypted before they’re sent.

With Continuity, you can also send and receive SMS and MMS messages in the Messages app on other iOS devices (with iOS 8) or a Mac (with OS X Yosemite), if you are signed in to iMessage with the same Apple ID as on your iPhone. 

Sign in to iMessage on an iOS device. Go to Settings > Messages, then turn on iMessage.

Sign in to iMessage on a Mac. On your Mac, open Messages, choose Messages > Preferences, click Accounts, then select iMessage in the Accounts list. Enter your Apple ID and password, then click Sign In.