In the US, an automotive platform is frequently referred to as an Automotive Architecture.
In the automotive world, from a consumer's point of view, a "platform" is a set of characteristics shared between several vehicles.
From a manufacturer's point of view, it is the other way around: The platform on which a vehicle is built is the fundamental basics on which the rest of the vehicle can be built. This can be compared with building a house: If you build a foundation and then stop building, a skilled architect can turn the building into a whole array of different buildings, limited only by the limitations of the foundation.
Originally, an automotive platform would consist of the floor pan (engine bay + the steel structure on which the rest of the vehicle is built), with suspension and driveline.
Currently, however, the definition has started to blur - as more and more manufacturers make parts for each other, it is starting to be difficult to tell which families of platforms still belong together. Volkswagen Passat, for example, shares the platform with Skoda Superb: same suspension, same floorplan, and even the same engines. Not strange, perhaps, when keeping in mind that Skoda now is fully owned by Volkswagen. More interestingly, however is to observe that the swedish-desinged Volvo V40, is built on the same platform as the japanese-designed Mitsubishi Carisma. Similarly surprising is that the Jaguar X-type shares a platform with the Ford Mondeo, that the Dodge Viper is on the same platform as the Dodge Dakota, and that the Chrysler PT Cruiser shares most fundamental characteristics with the Chrysler Neon. The new generation Chrysler Neon shares the same platform as the Mitsubishi Lancer EVO - yes the famous rally cars.
Currently, then, the definition of a "platform", is where the general mount-points fall on a vehicle. When the mounting points fall on the same places (i.e the suspension and the car's body strengthening anchor points are the same), it means that the different vehicles can in theory be built on the same assembly line, thereby saving production costs.
On a more fundamental level, however, putting a lot of research and development into the fundamental levels of car design means that once a manufacturer has a good foundation, they can build a whole series of vehicles on the same platform. These vehicles will not only share many of the same components, which can then be mass-produced cheaply, but they will also share crash characteristics and other safety-related issues. It also means that once the basics are in place, manufacturers can concentrate on licencing the platforms to other manufacturers, or create a small army of vehicles built on the same platform.
Volkswagen's relatively new PQ35 platform, for example, is the basis of their New Beetle, Golf, Golf Plus, Bora and Passat vehicles. It also houses a series of completely different vehicles, such as the Skoda Octavia, the Audi TT, the Audi A3, the Seat Altea and the Seat Toledo.
As the automotive industry progresses into heavier co-operation and more competition, new platforms will be developed, but in today's mass-industrial world, buiding vehicles from scratch, with different underpinnings, is just not an option.