From LightWiki

Reposted from the NewTek forum with permission from Weepul

First of all, the definition of a "spot":

The pinpoint location on geometry that is being sampled by any ray. For example, if a camera has no AA, each pixel is one ray and the exact location that's hit by that ray is a "spot" and the Spot Info node will give info about that "spot".

Object Spot: the coordinates of the spot relative to the object file. Example: a 1 meter cube that was shrunk to <0.1, 0.1, 0.1> in Layout would still output a value up to <0.5, 0.5, 0.5> at the corners.

World Spot: the coordinates of the spot relative to the scene's world coordinates, after the object has been moved, rotated, resized, deformed, etc.

Spot Size: an approximation of the area the spot represents. Even though each spot is infinitely small, it often would still represent an area, like a pixel. I don't know how that's calculated and is mostly useful for programmers making procedural textures.

Ray Source: the coordinates that the incoming ray came from in world space. Examples: if the camera has no DOF, the Ray Source output would be the location of the camera for each of the rays the camera uses to trace the scene. For an object seen by a reflection, it would be the location of the point on the reflective surface that can see this spot.

Ray Length: how far the ray went; the distance from the Ray Source to the World Spot.

Ray Direction: the normalized direction vector of the incoming ray.

Geometric Normal: the unsmoothed, unbumped surface normal vector of the geometry at that spot in world space.

Smoothed Normal: the surface normal vector of the geometry at that spot including any surface smoothing, but ignoring any bump, in world space.

Normal: the surface normal vector of the geometry at that spot, including surface smoothing and bump mapping.

Incidence Angle: the cosine of the angle of incidence between the ray and the surface normal. Equivalent to the dot product of the Normal and the Ray Direction, multiplied by -1. Same thing as the Gradient > Tools > Incidence node's default settings.

Incoming Eta: the refractive index of the incoming ray, so for example if you have an object embedded in glass with IOR 1.5, this value would be 1.5 when that object is seen through the refraction. Plain rays have the IOR of vacuum, which is 1. If a ray exits a glass object, it would return to 1.

Polygon Side: outputs a value of 1 if the ray sees the front of a polygon, and 0 if it sees the back (eg. when using Double Sided.)

Bounces: the index of the current ray bounce. Initial rays from the camera are bounce 0, then for example after one reflection it would be 1, then if it reflects again it would be 2, etc. Does not change for diffuse bounces of radiosity rays.

Preview: outputs 1 if the incoming ray is sent by a "preview", basically VIPER and the little previews in the node editor and surface editor. Otherwise, outputs 0.

Shadow Ray: 1 if the incoming ray is a shadow ray, 0 if not. Example use: making an opaque object cast a partially transparent shadow or a tinted shadow, for special effects.

Sampled Ray: 1 if the incoming ray is a "sampled ray", 0 if not. "Sampled rays" are typically any ray that's part of a group of multiple rays. For example, if a surface appears in a blurry reflection that's set to send out 8 rays, this will let you know. Blurry refraction and radiosity rays are, I think, the only other "sampled rays". Maybe the shadow rays for area lights/linear lights/etc. also - I don't know.

Radiosity Ray: 1 if the incoming ray is from radiosity, 0 if not.

Preprocess: 1 if the incoming ray is from preprocessing, 0 if not. Preprocess rays are cast for the previews during preprocessing of radiosity or SSS, or interpolated reflection/refraction.