the different shapes are called phases and they change as the moon moves round the earth and therefore changes the distance from the sun so the dark part is the shadow of the moon and the part you can see is the part in the sunlight.
Additional answer
The moon doesn't actually change shape. You may be asking why it sometimes looks round, at other times, crescent shaped. It's not changed shape, it's that you can only see part of it, and different parts at different times. When the position of the moon, earth and sun are such that they are in that order, the whole side of the moon that's towards the earth is lit by the sun and we see it as a round shape. But usually the earth is not exactly in between the moon and the sun, but a bit offset. We then see part of the moon that's not lit as well as part that is. The part that is varies in its apparent shape depending on just how much of it we can see. If most of what we can see is unlit, the lit bit looks like a slim crescent.
Also, the phases do not change as the moon moves round the earth but how its relationship with the sun and earth changes
The Moon
The moon's shadow changes its placement due to the moon's orbit around the Earth and the Earth's rotation on its axis. This movement causes the moon's shadow to shift and change its position relative to the Earth.
The sun
the way the sun light hits it during the rotation of the moon
The moon and it's gravitational pull
The density I beleive
The lunar orbit, or maybe more precisely, the mutual earth-moon orbit.Orbit
the sun because the moon doesnt make its own light
The moon causes the sea to cycle through tides by tugging the water into sort of an oblong circle and spinning it around the earth. The Sun causes the moon to change "phases" much as it causes the Earth to change days. The moon just turns much slower than the Earth and ends up never actually turning its "face" away from us.
The moon does not actually change shape, but its appearance from Earth does. This is due to its orbit around the Earth, which causes different portions of the moon to be illuminated by the sun at different times.
Yes, but different portions (size of area) of the moon reflect sunlight throughout the moon cycles which causes the moon to change shape.
The shadow is caused by the earth blocking the path of the light from the sun casting shadow on the moon. When the earth is not in between the sun and the moon then we have a "full moon."