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
JAR 25 covers Transport category airplanes (not excluding turbo-props)
In more general terms, one might say that JAR23 are Single Pilot Aircraft (SPA) whereas JAR25 covers Multi Pilot Aircraft (MPA).
Pilots with >500 hrs MPA gain credit in certain circumstances in JAR-FCL-1, such as MCC credit, etc. That might be one reason why some airlines now specify JAR25-experience. (Note that a change to FCL has been adopted, including, under certain conditions, time flown in multi-crew JAR-OPS-compliant operations on SPA as equal to JAR25-time. This however, doesen't mean that the airlines aren't free to set whatever reqiurements they wish)
cheers,