roxy,
A RFM is a rear finish mower that mounts on the three-point hitch of a tractor. It is a mower designed to mow a yard or lawn. A rotary cutter is a heavier mower with blades made to swing back if they hit a solid object. A rotary cutter is designed to cut not only grass but also brush. One of ways a rotary cutter is rated is by the size of brush or small trees that it is capable of cutting down. This is a pretty simplified explanation, but should give you an idea about them. A finish mower will give a better cut, but won't cut brush.