iMessage is exclusive to iPhone users. It's great because you won't use up your texts (if your plan has limited texts) since it uses Internet instead of your carrier network, you can group chat, you can tell when the message has been delivered, and you can send multimedia like video, images, and GIFs easily.
It's helpful to know when you're not using iMessage, so you can keep a close eye on your SMS limits. In a thread, if you see green bubbles, that means you are not using iMessage and texting someone who has disabled iMessages or uses an Android or Windows Phone device.
Photo: Nicole Nguyen