Pylanton, The stroke is the movement up and down of the piston, it can be measured in inches or mm. A long stroke engine will make it's power at a lower rpm and will make more torque at this engine speed. A short stroke engine will make it's power up high in the rpm band (less torque, more raw horsepower) because it has less distance to travel from the top to the bottom of the stroke, hence higher rpm. Listen to a Harley(long stroke twin cyl) verses an inline four crotch rocket(short stroke) and you will hear the difference!
Most long stroke engines are for slower scale flying with lots of pull where a short stroke is for more speed and higher performance.
I hope this helped. Fly High.
Suptter