Prior to this time aircraft were built up from an internally framing, typically of wood, which was then covered (or skinned) with fabric to give it an aerodynamic surface. The materials vary; some builders had experimented with the use of metal tubing for the framing, and a number had used thin metal sheets or plywood for the skinning. In all of these designs the idea of structure vs. skinning remained.
By the late 1920s the price of aluminum started dropping considerably and many manufacturers started using it to replace both the internal framing, and in some cases, the external skinning. A classic example of such a design is the Douglas DC-3, which is an "old style" plane built of new materials. The structure of the plane consists of a latticework of U-shaped aluminum beams, with a thin skin of aluminum rivetted on top.
When these designs started appearing it was realized that the skinning itself had significant structural properties of its own. With a sufficient thickness, one could do away with all of the internal structure. However this would be even heavier than the framing would have been. At thinner gauges the skinning could easily provide the structure for tension and shearing loads (metal resists being pulled apart quite well), and if it was bent into a curve or pipe, it became quite strong against bending loads as well. The only loading it could not handle on its own – at least in thin "skins" – is compression. Combining this sort of structural skin with a greatly reduced internal framing to provide strength against compression led to what is known as "semi-monocoque".
The result was a structure that was just as strong as ones made with older methods, but weighing considerably less. For aircraft construction this is a very important consideration. At the beginning of WWII the technique was just starting to appear, and many aircraft still used mixed construction. By the end, all planes were monocoque.
In the post-war period the technique became more widely used in other areas. It is now used quite commonly in automobile construction as well. In this application it is common to see true monocoque frames, where the structural members around the window and door frames are built by folding the skinning material several times. In these situations the main concern is spreading the load evenly, having no holes for corrosion to start, and reducing the overall workload. Compared to older techniques where a body would be bolted to a frame, monocoque cars are less expensive and stronger.