The shapes of the moon over a 29.5 day period are known as its phases. They are the result of the moon being illuminated by the sun in different positions as it orbits the Earth.
There are eight distinguishable phases of the moon. In the new moon phase, the moon is directly between the sun and Earth, receiving no illumination on the side of the moon facing our planet. The moon then transitions through the waxing crescent, first quarter and waxing gibbous phases. The right half of the moon is increasingly illuminated during this period in the northern hemisphere, while the southern hemisphere sees these phases grow from the left half of the moon. The halfway point of the phases is the full moon, when Earth's satellite is fully illuminated. From this point, the illumination decreases through the waning gibbous, third quarter and waning crescent phases.