Do IMessages use cellular data or Wi-Fi?

I'm a bit confused to the difference between an IMessage (between two iPhones) and text messages. Can someone please explain? Do iMessages use cellular data or only Wi-Fi, when it's connected?

IMessages are sent using data. This can be Wi-Fi, or mobile data such as 3g or 4g. It's a bit like Kik Messenger or WhatsApp, but for apple devices.