The phases of a moon (or other body) result from the positional relationship between the light source (Sun) and the observer (e.g. - Earth). The light source illuminates half of the sphere (assuming) of the moon, and the observer sees that part of the illuminated half, that is toward him.
If the illuminated half is away from him (i.e. - moon between light source and observer), then he sees a dark disk (e.g. - New Moon). If the illuminated half is toward him (i.e. - observer between light source and moon), then he sees an illuminated disk (e.g. - Full Moon). If the illuminated half is to one side, then he sees a partly-illuminated disk (e.g. - Crescent Moon, or Half Moon, or Gibbous Moon, in order of increasing amount that the illuminated half is toward him).
This assumes that the moon is out-of-line, with respect to the light source and observer - which is usually the case, for Earth and its Moon; if all three objects - light source, moon and observer - are aligned, then a New Moon will also produce a solar eclipse (moon obstructs observer's view of the Sun), and a Full Moon will coincide with a lunar eclipse (observer obstructs the light source's illumination of the moon, producing a shadow on it). The out-of-alignment condition is a result of the Moon's orbital plane being slightly inclined with respect to Earth's orbital plane (the Ecliptic plane), so that eclipses occur only during a time of year when there is a coincidental occurence of a New or Full Moon, at the same time that the Moon is at a point where its orbital plane, and the Ecliptic plane, intersect; usually, the Moon is instead slightly above or below the Ecliptic plane, during a New or Full Moon, so no eclipse.
You can draw a picture to visualize it all. Put the observer in the center, and the light source on one side, and have the moon orbit the observer (Earth's Moon orbits Earth, counterclockwise, viewed north-to-south; this is the same direction as Earth's rotation, and of the orbits of all planets in our solar system; it's called the right-hand rule, since it curves like the fingers of one's right hand, when the thumb points northward). Draw the moon in various positions in its orbit, and see how it would appear to the observer, with the illuminated side always toward the light source. This can also help to recognize at what approximate times of day the moon will rise and set, depending upon its phase.
If there are several moons, then what is described above, will apply to each moon, independently, depending upon its position, relative to the observer and light source. Thus, moons would appear in the same phase, only if they were in the same (angular) position.
