How much of the moon is lit up as it makes its revolution around the earth?

Only half of the moon is lit up as it makes its revolution around the earth.

The surface of the moon reflects the light incident from the sun and that is what makes the moon bright and shiny. While making its revolution around the earth, whatever is its position in the trajectory, only half of its entire surface is facing the sun and thus only half of the moon is lit up; the other half is always on the shadow side.  

What accounts for the differed brightness and shapes as observed from the earth is its position with respect to earth and the sun and this changes each day. When the sun, the earth and the moon all are in a straight line, the moon appears to be the brightest giving a Full moon and when it is between the sun and earth, the lit side is facing opposite to earth and thus it is not visible to us; giving rise to a New moon.