The phases are caused by the Earth's shadow (lit from behind by the sun) casting a piece of the moon into shadow. But because the moon orbits the earth those phases are only experienced the same way on one fixed part of the earth's surface. It's is the earth's changing orbit in space which really makes it seem as though the moon cycles through phases from a fixed position on earth. In other words, on the moon, in an earth day, you would actually cycle through all of the phases. Assuming you were on the earth-facing side, you would experience a strange type of day with the blocked out sun rising and setting behind either the earth or the moon twice a day. You would thus have two 'days' and two 'nights' in an earth day. On the 'dark side' of the moon, you would experience full day all of the time and no phases at all.
