surface chemistry
Pour water onto a clean glass plate and it spreads into a thin film; pour it onto a waxed car hood and it beads up and rolls off. Nothing about the water changed — only the thin skin where it meets the solid did. Surface chemistry is the study of that skin: the special chemistry that happens right at the boundary between two materials, which can behave very differently from the bulk on either side.
More precisely, surface chemistry studies the structure, reactions, and properties of the outermost layer of atoms or molecules of a material, and what happens when another phase (a gas, liquid, or solid) touches it. Atoms at a surface are unusual because they have neighbours on one side only, leaving them with unsatisfied bonds and extra energy. That hunger to bind is what lets surfaces grab passing molecules, speed up reactions, and hold things together.
This matters far beyond the lab. The catalytic converter in a car, the way a drug binds to a cell, how a battery electrode works, and why detergent lifts grease are all surface chemistry. The honest caveat is that surfaces are hard to study: they are vanishingly thin, easily contaminated by a stray fingerprint or a whiff of air, and often hidden under the bulk material, so much of the field is about getting a clean, well-defined surface in the first place.
A car's catalytic converter is a honeycomb coated with platinum and palladium. Exhaust gases stick briefly to the metal surface, react there into harmless products, and let go — pure surface chemistry doing the cleanup.
The outermost layer of atoms does chemistry the bulk material cannot.
Do not confuse a surface with an interface. Loosely, "surface" usually means a boundary with a gas or vacuum (the surface of a metal in air), while "interface" is the boundary between any two phases, such as oil meeting water. Surface chemistry is really interface chemistry under a more familiar name.