Early combines, some of them quite large, were drawn by mule teams and used a bull wheel to provide mechanical power. Tractor-drawn, PTO-powered combines were used for a time. Today's combines are self-propelled and use diesel engines for power.
Combines are equipped with removable heads that are designed for particular crops. The standard head, sometimes called a grain platform, is equipped with a sickle bar mower, and features a revolving reel with metal or plastic teeth to cause the cut crop to fall into the head. A cross auger then pulls the crop into the throat. The grain platform is used for many crops, including grain, legumes, and many seed crops.
Wheat heads are similar except that the reel is not equipped with teeth. Some wheat heads, called "draper" heads, use a fabric or rubber apron instead of a cross auger. Draper heads keep the crop orientation uniform, feeding grain headfirst into the throat, which allows slightly more efficient threshing.
Dummy heads feature spring-tined pickups, usually attached to a heavy rubber belt. They are used for crops that have already been cut and placed in windrows.
While a grain platform can be used for corn, a specialized corn head is ordinarily used instead. The corn head is equipped with snap rolls that strip the stalk and leaf away from the ear, so that only the ear (and husk) enter the throat. This improves efficiency dramatically since so much less material must go through the cylinder. The corn head can be recognized by the presence of points between each row.
Occasionally rowcrop heads are seen that function like a grain platform, but have points between rows like a corn head. These are used to reduce the amount of weed seed picked up when harvesting small grains.