The phase of the Moon refers to the shape of the illuminated (sunlit) portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun. The half of the lunar surface facing the Sun is always illuminated, but the portion of this hemisphere that is visible to an observer on Earth can vary from 100% (full moon) to 0% (new moon).
When the Sun and Moon are aligned on the same side of the Earth the Moon is 'new', and the side of the Moon visible from Earth is not illuminated by the Sun. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress from new moon, crescent moon, first-quarter moon, gibbous moon and full moon phases, before returning through the gibbous moon, last-quarter (or last quarter) moon, crescent moon and new moon phases.