The moon has phases with distinct appearances because it is constantly rotating around the Earth and being illuminated by the sun at different angles. The most common phases are the new moon, first quarter, full moon and last quarter.
The moon is constantly in orbit around the planet and completes a full revolution approximately once a month, or every 29.5 days. The sun is able to shine light on half of the surface area of the moon at any one time. When the moon is directly between the Earth and sun, the moon cannot be seen from the Earth, which is the new moon phase. Similarly, when the moon is on the opposite side of the Earth in relation to the sun, it will be in the full moon phase as all the light bouncing off the moon can be seen from Earth.