Participating media is a form of indirect illumination where light interacts with particles that occupy a space, rather than with a surface. In Maya, photons interact with volumetric effects that participate in the transport of light by absorbing some of the light.
Example of participating media are shafts of light in a dusty barn or the glow around beam of light from a car headlight.
Participating media works similarly to the way global illumination works on surfaces, except that volume shaders and volume photon shaders are required.
Participating media can disperse volumetric caustics too.
