Since the moon rotates once on its axis every 27.32 days, but also circles the earth in the same amount of time, we only get to see one side of it. Therefore, the "dark side of the moon" is not really dark (like the earth, it reflects the sun;) it's just that there is a whole side we never get to see from Earth.
The moon exhibits different phases as its position relative to Sun and Earth changes. It appears full when the sun and moon are on opposite sides of the earth, and becomes invisible, or New Moon, when the earth blocks out the sun's light preventing it from shining on the moon. The period of time between two Full Moons is 29.5 days. Since this is about 2 days longer than the time it takes the Moon to orbit the Earth, the day that it appears full does not always correspond with the same calendar day.
The lunar phases (crescent, half, full, half, crescent) are not created by the shadow of the Earth on the moon. They are due to our being able only to see part of the illuminated face of the Moon at a time. Therefore, in the Northern hemisphere when the right side of the Moon is dark, the light part that reflects the sun is apparently shrinking. We say the Moon is waning (shrinking, moving towards New Moon.) Similarly, when the left side is dark, we say the Moon is waxing (increasing, moving towards Full Moon.)
