You may have noticed some of your iPhone text messages have either a blue or green background.
A blue background means that the message exchange is happening via iMessage with another iOS device (an iPhone, iPad, iPod Touch, Mac, etc). Text message rates don't apply, and these messages can be send through your cellular provider's data network or via WiFi (no cell service needed).
A green background means that the message is being exchanged with a non-iOS device (Android, Windows phone and so on) and was delivered via SMS through your mobile provider. Messages can only be exchanged with these recipients when you have cell service.
If all your messages are green, it could mean that iMessage has been disabled on your device Navigate to Settings > Messages and enable iMessage.