Psionics are the art of tapping the mind's potential. Where users of magic (both arcane and divine) harness external energies, psionicists shape, control, harness, and utilize the mental forces that infuse their own being. Their effort is focused inward rather than outward. This type of knowledge often comes from long and intense meditation coupled with physical extremes, though some creatures have untrained psionic powers (sometimes called wild talents).
In the planes, few distinguish much between magic and psionics despite their great differences. Many consider psions to be just another type of wizard. Even Githyanki, experts in the field, call psions, sorcerers, wizards, and warlocks all "warlocks" regardless of the source of their powers. The Githzerai train sorcerers, psions, wilders and other mystical classes in their Floating City. Psionicists themselves know their powers aren't magic, but others are usually more concerned with how these powers are going to hurt them or hurt their enemies than worried about what exactly they are. Psionic effects normally are affected by planar conditions (for example, the Spire) the same way magic is.
Illithids have great disdain for magic, and mind flayer arcanists are exiles among their people.