A backdrop object is a plane surface that automatically rotates itself perpendicular to the camera in ray trace rendering and scales itself to fill the image. It defines UV coordinates which run from (0,0,0) at the top left corner to (1,1,0) at the bottom right corner. A backdrop is a real plane surface whose distance can be adjusted using the Distance control on the property window's object specific tab.
A backdrop plane, such as all other geometric objects, can be textured (and shaded in general) using VSL materials. The distance can be modified using a ray hit shader, the backdrop can be translucent etc. Backdrops have some special object attributes set by default:
They are self-illuminating (Property window's Color tab/Illumination channel)
The diffuse shading is turned off (Color tab/Color channel)
They do not cast or receive shadows (General tab)