Twinsun is a planet, and it's rotational plane has stabilized between two suns, creating two constantly-lit hemispheres (no nights - when do they party?) and an icy mountain range in the middle.
The Northern hemisphere has been evacuated by Dr. FunFrock, to hide his search for the Well of Sendell.
The planet has a lot of islands on it:
There's also a few other notable sites:
The Little Big Adventure series of games plays on this planet.