Chicken Leg is roughly correct, but to be more specific, we are talking about the differences between JAR 23 and JAR 25 aircraft (soon to be EASA 23 and EASA 25).
JAR 23 covers Normal/Utility/Aerobatic category airplanes w/ 9 or less PAX seats, MTOW < 5.670 kgs/12.500 lbs, and commuter category airplanes, propeller driven, w/ 19 or less PAX seats, MTOW < 8.618 kgs/19.000 lbs - the King Air falls into this category, also small bizjets such as CJ2.
JAR 25 covers transport category airplanes (not excluding turbo-props).
In more general terms, one might say that JAR23 are aircraft that can be flown single pilot for private operations, whereas JAR25 covers Multi Pilot Aircraft (MPA) for public transport operations. But like everything in this crazy world, there are exceptions, so for example CJ3 is JAR25, but can still be flown Single-Pilot on private operations and CJ2 is JAR 23 but has to be flown multi-crew for public transport.