We see the phases of the Moon because the Moon orbits the Earth, and as it does so, we observe different portions of its sunlit half. The Moon does not produce its own light; instead, it reflects sunlight. As it moves along its orbit, the relative positions of the Moon, Earth, and Sun change, causing the visible illuminated fraction of the Moon to vary from fully dark (New Moon) to fully illuminated (Full Moon) and back again over a lunar month (about 29.5 days).
The phases occur because only the half of the Moon facing the Sun is lit, and depending on where the Moon is in its orbit, we see varying amounts of that lit half. For example, during the New Moon, the side facing Earth is dark, while during the Full Moon, the entire face visible from Earth is illuminated. The phases include New Moon, waxing crescent, first quarter, waxing gibbous, Full Moon, waning gibbous, last quarter, and waning crescent.
Thus, the changing appearance of the Moon is due to our changing viewpoint of the sunlit portion of the Moon as it travels around Earth, not because the Moon itself is changing shape or light emission.
