Most headphones use dynamic drivers — a tiny speaker cone moved by a magnet and voice coil. Planar magnetic headphones work differently: a thin membrane with a printed circuit is suspended between magnets and driven across its entire surface. The result is lower distortion, faster transient response, and a different kind of resolution.
The tradeoff? Planar magnetics tend to be heavier, harder to drive, and more expensive. But at the same price point, they often resolve more detail.