Sometimes your message might be sent as an SMS/MMS rather than an iMessage, perhaps because your recipient does not have an Apple device or an internet connection. To distinguish between the two:
iMessage: iMessages are sent and received over the internet, using your Wi-Fi or cellular data connection. iMessage messages are displayed in blue text bubbles.
SMS/MMS: SMS and MMS messages are sent and received over the cellular network. SMS messages are text messages, while MMS messages can also include photos and videos. SMS and MMS messages are displayed in green text bubbles.