The two terms are fixed shaft, and free turbine. They both relate to the form of construction of the engine. On a free turbine engine, the compressor/turbine assembly rotates independantly to the prop/gearbox/turbine assembly. That is, you can spin the prop, and the compressor wont turn. (For example PT6). On the fixed shaft, the compressor turns when the prop turns, because they are on the same shaft (through a reduction gearbox of course)(for example Garett 331). I hope that clutch comment was a wind up!
Hope this helps.