Why do phases occur?
As it circles Earth, the Moon has phases, which allows the portion we see illuminated to change. It takes 27.3 days for the Moon to orbit Earth, but 29.5 days is the lunar phase (from the new Moon to the new Moon). The Moon rises as the Sun sets, with the side facing Earth entirely open for sunlight.
What are these phases?
New Moon: Completely dark
Young: The beginning of a lunar month
Waxing: The illumination of the moon is growing
Crescent: Less than half of the moon is illuminated
Quarter: Exactly half of the moon is illuminated, and half of it is in darkness
Gibbous: More than half of the moon is illuminated
Full Moon: The entirety of the moon is illuminated
Waning: The illumination of the moon is decreasing
Old: the end of a lunar month