The Inner Planes include the four main Elemental Planes that everyone knows (Air, Water, Earth, Fire), and also the Positive Energy Plane and the Negative Energy Plane.

What only sages know is that there are regions between those planes, where their properties mix, that can be considered planes on their own: the Paralemental Planes (where two elements mix, becoming an intermediate of sorts), and the Quasielemental Planes (where an element mixes with positive or negative energy, becoming "more" or "less" than its base element).