One side of the moon is always illuminated by the sun. When the earth is between the moon and sun we see the illuminated side, full moon. When the moon is between the sun and earth we see the dark side, new moon. When the moon is 90 on either side we half of the dark side and half of the illuminated side, quarter moon.
The exception is when the earth is directly between the sun and moon, essentially blocking the suns line of site of the moon which would be a lunar eclipse.