I don't quite understand how this is different from path tracing (except for maybe that it comes from the light source) or why it helps...

@anonymous Caustics are patterns formed on diffuse surfaces as a result of light reflecting/refracting off of specular surfaces. For example, next time you get a cup of coffee in your favorite mug, check out the caustic patterns that get formed at the bottom of the mug when under a point light source. The light paths that form caustics have the following form: L -> S+ -> D -> E, where L refers to the light source, S+ refers to one or more specular bounces, D represents a diffuse bounce, and E is the eye.

Rendering caustics tends to be very challenging for standard Monte Carlo techniques. In your standard path tracer, we start by shooting out rays from the eye to a diffuse point in the scene. It is then necessary to connect this diffuse point to a light source through a series of specular reflections, and the probability of sampling these paths is very low (resulting in noisy renderings).

Photon mapping starts by shooting out photons from the light source, and storing those photons at diffuse points in the scene (the photon map). These photons concentrate at diffuse points where the caustic is brightest. We then perform a rendering pass similar to our standard path tracing approach of shooting out rays from the camera, and then sampling the photon map at a diffuse point where the ray intersects the scene. This tends to be much more efficient (produces better quality images with fewer samples) than a classic path tracer.

Also, a great reference for photon mapping is the following book: "Realistic Image Synthesis using Photon Mapping", by Henrik Jensen.

So would a renderer use some combination of these, e.g. Monte Carlo for most of the scene and then photon mapping for caustics?

this image is so beautiful..