The planets, as viewed in the sky, exhibit characteristic aspects and phases. "Aspects" refers to the location of the planet with respect to our overhead sky reference (objects on the celestial sphere); "phases" refers to the fact that the planets, through a telescope, exhibit phases (differing amounts of lighted hemispheres as viewed from the earth). The terminology associated with these aspects and phases is different, depending on whether we refer to an inferior planet or a superior planet.
Gibbous phases are phases between quarter and full phases. Greatest Elongation refers to the largest separation of the planet from the Sun in our sky, either to the East, or to the West. Thus, we see that the inferior planets exhibit a complete set of phases (just like the Moon) as viewed from the earth, and can never be further from the Sun than the angles defined by greatest elongation.
Aspects and phases of the inferior planets.
When a superior planet is at quadrature, it is on our celestial meridian at sunrise or sunset. Comparing with the preceding diagram for the inferior planets, we notice two basic differences: (1) The superior planets do not exhibit a full range of phases; they are always gibbous or full. (2) The superior planets can be located at any distance East or West of the Sun in our sky, unlike the inferior planets where there is a limiting angle away from the Sun (greatest elongation).
Shockwave movie illustrating aspects and phases of the superior planets.