Fuselage is originally a French word, meanig the main structure of the aircraft, which holds both human and cargo, depending on the type of the aircraft.
The three most comon types of fuselage are:
1. Truss or framework type:
This consists of light gauge steel tubes which form a frame triangular shape to give the most rigid of geometric forms. Each tube carries a specific load, the magnitude of which depends on whether the aircraft is airborne or on the ground.
This type of fuselage is commonly found on the first few generations of aircraft. They are strong, moderately easy to manufacture, but did not necessarily implement the concept of aerodynamic.(toandfromtheairport.com)
2. Monocoque Construction:
‘Monocoque’ is a French word meaning ‘single shell’. All the loads are taken by a stressed skin with just light internal frames or formers to give the required shape.
To be a ‘true’ Monocoque the structure would have no apertures at all.
Although it practicaly can carry more load, the drawback of this type is that it may require maitenance more compared to the other designs, as the sturcture needs to be reinforced in order to maintain the structural integrity.(machinedesign.com)
3. Semi-Monocoque Construction.
As aircraft became larger, the pure Monocoque was found not to be strong enough. Designers came with a new concept to make fuselage stronger; the Longerons run lengthwise along the fuselage joining the frames together (see picture below for more detail). The light alloy skin is attached to the frames and longerons by riveting or adhesive bonding. Doublers are required when cut-outs are made to provide access panels, doors or windows. Bulkheads isolate different sections of the aircraft, for instance the engine compartment from the passenger compartment. Bulkheads are of much stronger construction than frames or formers, as the loads upon them are so much greater.
This concept is widely used both in military and also in the commercial industry. In military, this concept is believed to enable planes to gain more speed.