Assigning a shape to a light modifies the illumination of a scene.
The "area" parameter on the light is a property of a light. Just as a light can have a color, it can also be assigned a shape. For example, you can shape it like a rectangle so that it acts like panel lighting in a ceiling. Or you can shape it like a line, so it acts like a narrow fluorescent light tube. The area light is a way to assign a shape to the light. The shape affects the rendering and shadows in the same way that a panel light casts different light than a tube light in the real world.
The shape property is available in the palette. This property will appear based upon the type of property selected. Under the Render Shadow Detail panel, there is a Type property. This specifies the type of shadow cast by the light. The following types are available: Soft (shadow) map, Sharp (default), Soft (sampled). By selecting the Soft (sampled) option, the Shape property becomes available.
The shapes offered depend on the type of light. You can select the Type property for the light distribution under the General panel in the Lighting Properties palette. If Spotlight and Soft (sampled) are selected, then the Shape types available are Rectangular and Disk. If Point and Soft (sampled) are selected, then the following shapes are available: Linear, Rectangular, Disk, Cylinder, and Sphere. If Web and Soft (sampled) are selected, then the following shapes are available: Linear, Rectangular, Disk, Cylinder, and Sphere. The samples property on the area light can be used to control the trade-off between rendering time and shadow accuracy.
The Visible Render property found under the Rendered Shadows Details panel controls the visibility of the shape in the rendering of the scene.