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From Odwiki

Backwards RayTracing is sometimes called forward RayTracing.

The first ray-tracers would send rays from the camera (eye) through the screen and into the scene. These rays would intersect geometry and a surface shader would query the illuminating point lights. This technique is commonly known as forward RayTracing (though some call it backwards ray-tracing).

However, this technique doesn't allow for easy computation of some LightPaths.

Consider a magnifying glass which is focusing light onto the surface, causing Caustics. When the renderer is shading the surface, it's only knows about the position of the light source, it doesn't know that light gets bent between the light source and the surface.

Backwards ray-tracing attempts to solve this problem by sending rays from the light sources.

There are several different techniques including Metropolis Light Transport, PhotonMapping, and Bi-Directional Ray-Tracing.