Generally a cold frame, small or large, is a structure with open ends. Often designed so that if the temperature plunges, the open ends can be closed. Often, with some crops, this is all that is needed. Most definitions of a greenhouse, would at minimum include all sides and roof covered against temperature extremes. Much like homes, greenhouses used in Nursery applications, or for commercial growers, uses climate controls to offset extremes in temperature, control humidity, and move the air inside, acting as artificial wind. Without fans to move the air robustly, tall spindly starts that fall over outside follow. Stagnant air leads to problems just like in your home. An unheated greenhouse however can be used for the fall and spring.