Monocoque
Monocoque fuselages use a rigid outer skin attached to a series of bulkheads that are supported by stringers. The rigid skin is considered "stressed' because it is inflexible and is typically made of wood or metal. The skin on a monocoque fuselage handles all the stress on the body of the fuselage, not the supporting bulkheads structure.
Welded Truss
A welded truss fuselage uses metal tubes (longerons) along the whole length of the body welded to supporting metal tubes to form the frame. The frame absorbs the various stresses the body is exposed to, allowing the skin to be constructed of light, flexible, composite or rigid materials.
Semi-monocoque
Semi-monocoque fuselages use additional longitudinal support members between the bulkheads to allow more of the stress on the craft to be dispersed through the frame than in monocoques. This allows for less rigid and lighter skins to be used, decreasing the weight of the fuselage.
Primary Useage
Light, personal planes normally use a welded truss frame, as neither the weight of the cargo, the plane's speed, nor the maximum altitude that the plane reaches puts much stress on the fuselage. Helicopters, commercial airplanes and jets all experience much greater stresses in flight and use the monocoque or semi-monocoque fuselage constructions.