Caustics are light effects that caused by specularly (as opposed to diffusely) reflected or refracted light.

• Refracted caustics happen when specularly transmitted light bends through a transparent or semi-transparent object or volume onto a diffuse surface.
• Reflected caustics happen when specularly reflected light bends through a transparent or semi-transparent object or volume onto a diffuse surface.
  Note

  In the natural world, caustics are actually a form of global illumination, but they are treated separately by mental ray for Maya, making it easier for you to control them.