When the moon is full, the moon is at its brightest, and the entire disk is visible. New moons occur when the Earth comes between the moon and the sun, resulting in a moon that is completely obscured by the Earth's shadow and is barely visible in the night sky.
The moon goes through many phases in the course of a lunar month, which has 29.5 days in it, according to MoonPhases.info. The moon is said to be waxing when more of the sun's light falls upon it, leaving more and more of its surface visible. When waning, the moon travels further into the Earth's shadow, and visibility is decreased.