Auto body is the name we give to the portion of an automobile which gives it shape. The body is usually made of sheet steel which is heated and stamped into shape. Depending on the design of the car, the body may also be involved in providing structure and actually bears load when the vehicle is under stress. This is known as the unibody design, and has been in use since the 1930s, introduced by Volkswagen and Nash. Some vehicles, like the Opel Aero GT are actually full-monocoque, which means that the outer skin of the car bears load. While the outer skin of the car is simply sheet metal, what you don't see is structural rails like the rocker panel and roof rail, and the pillars (simply vertical boxed sections) which connect them.
The auto body is really what determines what kind of car it is, though the chassis (or frame) determines much of how it behaves on the road -- its handling. The body and frame work together to create the car itself. Besides unibody cars, we also have "full-frame" designs, which feature a chassis which traditionally runs from the front bumper to the back bumper. Full-frame cars are characterized by the car mostly bolting together, while unibody cars are welded. When you need to replace the inner or under fenders of the front end of a unibody car, you have to actually cut the car to get the front end off, which gives rise to the terminology, front clip, whereas on a full-frame car, you can simply unbolt the front end, known as a doghouse. More significantly however, on a full-frame car the chassis bears most of the load, and is intended to flex a significant amount. On a unibody car, the body bears most of the load, and is intended to be mostly rigid.
Most extremely large, long-wheelbase cars are full-frame, because it is difficult and expensive to make a unibody rigid over a long distance. Other than land yachts like the Ford LTD and just about any Cadillac, cars since the 1960s have been partial-monocoque. We call these stub frame cars. Generally speaking they have rails in the front, a partial ladder chassis, which holds the engine and the front suspension and runs approximately to the area of the front seats, while the rear of the car is a unibody with welded quarter panels instead of bolted-on rear fenders. In fact, it is not uncommon for the roof and the quarter panels to be the same piece of steel. Stub frames usually provide a larger well under the hood at the expense of weight.
Modern unibody cars, which came into vogue in the 1980s, are generally front wheel drive, with a transversely mounted engine sitting on top of the steering rack. The engine is generally held by two or more engine mounts which connect to sections of frame. On a full-frame car, this frame sits under the auto body, but in a unibody car, the frame rails are welded into the design. By using front wheel drive, one eliminates the drive shaft and the rear axle or axles, thereby significantly reducing weight. The typical unibody of today is assembled by machine and electrically spot-welded by robots.
Painting of auto bodies presents a number of unique challenges when compared to painting any static structure. A car is subjected to a broad range of temperatures, especially given that it generates a large amount of heat during its operation, which in turn will cause steel body panels to expand and contract. Cars tend to flex when driven, which will add additional stress to the paint. Different pigments and additives like metal flake (usually aluminum) and pearl (generally glass beads) sink in paint at different rates until it dries; Modern catalysed paints will dry fairly rapidly but will be delicate until they cure, which can take as little as a day, or as long as a month -- or forever if the temperature is too low. This is however a giant step upwards from the older lacquer paints which never cure, and act essentially like dried mud, though their thinner is lacquer thinner, not water.
Auto body is repaired through a handful of methods, but all of them return in some way to the basic idea of the hammer and anvil. There are things you can do to make metal shrink (and thicken), such as properly heating and optionally quenching it; Beating metal with a convex hammer will stretch it, and make it thinner. In most cases a tiny handheld version of an anvil called a dolly is used, along with a lightweight body hammer which has a broad, slightly convex face. It is also possible to drill holes along the low part of a crease and put hooks through it to pull it up, or to weld pins onto the body and grab onto them with a T bar or slide hammer and tap them up, but everything comes back to the principles of the hammer and the anvil. If the steel is still out of shape after being beat with the hammer, body filler can be used to smooth out ripples, waves, and dings, though not to properly patch holes, which requires welding. The welding is generally done with a MIG welder which is designed for low-temperature applications, and is easy to use - Anyone can pick it up in about half an hour. Holes can be bridged by using fiberglass-reinforced plastic body filler, but nothing approaches the strength of steel, especially tensile strength.
American and German auto body has generally been made of higher quality, heavier-gauge steel which is milder, whereas Japanese cars are made of recycled steel. This recycled steel is harder, which means you can use less of it to bear the same amount of load, but it also means that it is more likely to buckle when a load is applied in some unintended way, such as being struck with a baseball, a foot, or another car. Repairing Japanese steel is trickier because steel hardens as you work it (this is known, logically enough, as work hardening) and Japanese steel is already harder than American stuff. In addition, it is thinner, which means it's easier to grind a hole in it, or stretch it (hence making it thinner) in such a way that it cracks or tears. Most small American cars are now made out of the same recycled steel in order to keep both costs and curb weight down. Other notable metals include the stainless steel body of the DeLorean, and aluminum body panels as used on the Honda NSX.
In addition to the use of recycled steel, the auto industry has long used plastic body panels in an attempt to make damage-resistant cars and to reduce weight. Henry Ford prototyped a car in the 1940s made almost entirely of a composite of hemp fiber and corn plastic, down to major structural members. Probably the best known use of plastic panels today is the bumper covers of most unibody cars, which are typically made of polyurethane; Most auto enthusiasts are quite familiar with the plastic panels on the Pontiac Fiero, and of course many vehicles from GM-owned automaker Saturn feature plastic door skins. Funny cars and other race cars often utilize bodies made completely or partially from fiberglass, and they utilize a tube frame for weight and rigidity, the body then being responsible only for aerodynamics and appearance. It is also common to replace portions of auto body with carbon fiber pieces, usually starting with the hood and trunk lid, to reduce weight.