From BlenderWiki

What the Light Affects

Every lamp has a set of switches that control which objects receive its light, and how it interacts with materials.

Layer

Causes the lamp to only light objects on the same layer.

Negative

The light produced by the lamp is subtracted from the ones “available” on the surfaces that it hits, which darkens those surfaces instead of brightening them.

No Diffuse

Prevents the lamp from producing diffuse light (it doesn’t really “light” things).

No Specular

Prevents the lamp from producing specular highlights.