Seen from the Earth, the moon always looks different, sometimes a thin crescent, sometimes a half moon, It becomes larger and larger as the days go by, until it becomes a perfect circle. But then it becomes smaller, day after day, until it disappears. We call this change in shape the phases of the moon.

It takes 27 days for the moon to complete all these changes. Our modern calendar is based on these lunar phases. In fact the moon never changes it's shape. The light which the moon seems to give out actually comes from reflected sunlight. If, for instance, you put a ball in a dark room and then shine a pocket torch on it, you will see half the ball lit up; the other half stays dark. It is the same for the moon and the other planets. The moon is half in light, half in shadow.

From Earth we only see the lighted half of the moon. Most of the time we don't even see the whole of this half of the moon. The solar light is reflected from Earth to the moon, and vice versa.
However, when the moon is a thin crescent you can see, though faintly, the dark part. This is actually slightly lit by the solar light.