The phases of the moon are: new moon, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter and waning crescent. After the waning-crescent phase the moon returns to the new-moon phase and the cycle starts again. The complete cycle takes a little over 29 days.
The sun always shines on the half of the moon it faces. The other half is shadowed. The position of the Earth, moon and sun determine how much of the illuminated moon we see. During the new moon, the moon is between the Earth and the sun. This causes the sun to light the back side of the moon, relative to Earth. The side of the moon facing Earth is completely in shadow. During a full moon, the Earth, moon and sun are aligned but the moon is on the opposite side of the Earth from the sun. The entire sunlit part of the moon faces the Earth. During the first- and third-quarter moons, the moon is at a 90-degree angle to the Earth and sun, so that half of the visible moon is in shadow, with half in light. The moon is called a crescent moon when less than half of the moon is visible, and a gibbous moon if more than half is visible. A waxing moon is growing lighter and a waning moon is growing darker.
The transition from full to new moon takes approximately two weeks, and then the Moon spends two weeks waxing through the crescent, half, and gibbous phases until it reaches another full moon.