The best definition would be: Rotary- The crop material must pass around the circumfrence of the threshing mechanism. In other words it must work all the way around the "rotor" in order to move through the machine. The front section of the rotor does the agressive threshing while the rear section (without rasp bars) is supposed to spin the grain out. Conventional- the crop passes between a concave and a cylinder. The cylinder beats the crop against bars on the concave to thresh. The crop typicaly then passes onto straw walkers which shake the grain out of the crop mat. Although many years ago Class made a combine which was just a long bunch of cylinders and concaves and had no walkers in it. As far as which is the best- well that depends on who your talking to. Generally rotary combines will accept more material. However they also produce much more smaller material that the chaffer must separate. Also in wet crop conditions a single rotor combine develops a roping effect. This will prevent separation of the grain and can plug the rotor. This is normally just a problem in northern states and parts of Canada. In a conventional combine the machines limiting factor is the amount of crop mat that can pass through the cylinder area and be threshed. When you go to fast they plug up. On the other hand the limiting factor in most rotarys is the chaffer capacity to separate. A rotary can be pushed so fast that a solid mat will develop across the chaffer which will carry grain out the back. (especially in soybeans) This will carry right over loss sensors at the rear of the chaffer so if the operator is to lazy to get out and dig through the chaff trail he will never know he was lossing grain untill a few weeks later when there are strips of green growing in the field where the combine passed.