From BlenderWiki

This panel appears in the Shading Context panel group which is accessed using F5 or clicking button in the Buttons window. This panel can have slightly different contents depending on the the type of lamp selected in the Shading Preview panel. Only the differences relative to the Lamp type light are listed. There are five different type of lights:

[edit] Lamp type

• Ray Shadow - Use a ray tracing algorithm for generating shadows.
• Only Shadow - Cause light to cast only shadows with out illuminating objects.

[edit] Area type

• Samples: - Set the extra amount of samples taken in a 2D pattern. For example, NxN samples.
• Umbra - Emphasize parts that are fully shadowed.
• Dither - Use 2x2 dithering for sampling.
• Noise - Use noise for sampling.

[edit] Spot type

• Ray Shadow - Enables ray traced shadows.
• SpotSi - Set the angle of the spotlight beam in degrees.
• SpotBI - Set the softness of the spotlight edge.
• HaloInt - Set the intensity of the spotlight halo.
• Only Shadow - Let the spot cast only the shadow and no light. This further explained in Manual/PartV/Buffer Shadows: Tweaking Buffer Shadows.
• Square - By default Spot lights cast a cone of light with a circular cross-section. There are cases where a square cross section would be helpful or required. Selecting this option generates a pyramid shape volume rather than a cone volume.
• Halo - Renders spotlight with a volumetric halo. It is as if the light rays were passing through a hazy medium. This option is further explained in the Manual/PartV/Buffer Shadows: Volumetric Light section.

• Buf. Shadow - Configures spotlight to use shadow buffers. This can consume more memory during rendering depending on the shadow buffer size but can be much faster than ray traced shadows.

• Shadow Buffer Size - The size of the shadow buffer in multiples of 16.
• Clip Sta: - The boundary where shadowing starts. Objects closer to the light will not generate shadows.
• Clip End: - The boundary where shadowing ends. Objects farther from the light will not generate shadows.
• Samples - The number of shadow map samples.
• Halo step: - Set the sampling frequency for the Halo effect.
• Bias: - Set the sampling bias.
• Soft: - The size of the shadow sample size.

[edit] Sun type

The panel contents is the same as it is for the Lamp type light.

[edit] Hemi type

The panel's content is blank as the Hemi type light does not require them.