A planar is a denizen of one of the planes of the multiverse, except the Prime Material plane (q.v. primes). The term is typically used to refer to the denizens of the Outer Planes, but it can also apply to people from the Inner Planes, Astral Plane, or Ethereal plane.

A human is a human, regardless of their plane of origin, so it's not always possible to identify a planar on sight. However, by virtue of being born on the planes, planars have some qualities that primes do not:

• Planars don't have a silver cord on the Astral Plane.
• Planars can see any gate between planes as the glowing outline of a portal.
• Planars count as extraplanar beings for spells like protection from evil, holy word, and exaction, and can be hauled off to the Prime by monster summonings and similar spells.

References

• Planescape: Planescape Campaign Setting, p. 8-9 , A Player's Guide to the Planes