A diagram which demonstrates why the moon goes through phases. Click on image for full size (13K GIF)

When the Moon is between the Earth and the Sun (labelled 1), the side of the Moon facing the Earth is the dark side. The Moon cannot be seen. We call this the New Moon because it begins a new cycle of phases. When the Earth is between the Sun and the Moon (5), the side facing the Earth is the light side. We call this Full Moon, even though we can only see half the Moon at one time.

Halfway in between these times (3 & 7), only half of the near side of the moon is reflecting sunlight. So we can only see one-quarter of the Moon. We call these phases First and Third Quarters.

All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from Earth, and whether this part is going to grow or shrink.