Radios are entities which emit music, much like jukeboxes. However, they will only play the song from the music disc Valve - Radio Loop, repeating the song when it reaches its end. They can be turned on and off by right-clicking them, unless canRadioBeTurnedOff=0, which some people would say is annoying. They can be carried with G even without a Portal Gun. They can also be made indestructible by setting isRadioInvincible=1, which means that they cannot be broken down into item form by either punching them or suffocating them (moving them into a wall).
Relation to Portal
The radio is featured in both Portal games.