A body kit is a set of pieces which, when added to the body of an automobile, alter the appearance and aerodynamics of the vehicle. They are available for nearly every brand of car (though not all models), and come in many different styles, and from many manufacturers.
Body kits of various types and levels of complexity have been around for a great deal of time, but they first gained real popularity in the 1970s. Commonly applied to American muscle cars, and evolved from the designs of race cars, they gave the vehicles a meaner and more aggressive look, while not coincidentally reducing the amount of lift generated by the car. Some spoiler designs, however, actually increase turbulence, and thus lift, beneath the car. Significant body variations in production automobiles in the 1960s probably led to the first commercial body kits, to make cars of lower trim levels look like their more expensive sister models.
Today, body kits are made for all classes and kinds of cars, from the Mazda 323 to the Porsche 944. They are made of all kinds of materials, especially fiberglass and carbon fiber, but also a wide range of polycarbonate plastics. Some of them are sold only as a "four pack", which includes front and rear ground effect spoilers as well as side skirts, with rear wings or spoilers sold seperately, or some may be available as individual parts, allowing you to combine parts of different kits. In addition, it is possible to buy features like fins, scoops, and flares to pad out cars without a full kit, or to change the look of an existing kit.
Generally speaking, body kits are provided unpainted, though they are usually sanded and then either primed or gel coated. Most manufacturers' kits fit fairly well, but some will need some trimming (generally with a small circular saw or hack saw) before installation. The kits are bolted into place on the original body work, sometimes replacing fascias or bumper covers, especially in the front. They are generally painted to match the car, or painted with the car, after being installed, so that the fit can be checked, and to avoid marring during installation.
It is also possible to make custom body kits for cars. Many companies specialize in this, and if you bring them a vehicle they will make you a kit for anything. The price varies based on size, complexity, and who you talk to, but prices are generally significantly higher than all but the most expensive premade body kits.