Eternia was the planet that He-Man and the Masters of the Universe took place on. Eternia was roughly a roughly Earth-type planet, class-M if you will.

Important landmarks: Castle Greyskull, home of the Sorceress
Snake Mountain, lair of the evil(and incompetent) Skeletor
Castle Eternia, capital of the nebulously defined kingdom that Prince Adam's father, King Randor(thanks to Stride for the name. It completely slipped my mind.), ruled.