WiFi is the medium of connecting devices wirelessly, so yeah, pretty much radio. Like walkie talkie style between devices, or most of the time, one of your devices and your router which actually carries the internet connection from the wall and passes it to such devices in a basic sense.

When you're outside of your house the internet you get on your phone is not (usually) carried by WiFi but is instead through a cellular network using the same (or similar) tech that is used when you have a phone call or send text messages. But once again instead of sending phone call data or text messages, it streams an internet connection to your phone.

I know you didn't actually ask for an explanation but I thought it would be fun to try and write it out anyway.