These parameters control the scattering of light by the participating medium (fog, smoke, etc).
Increases/decreases the atmospheric volume density. The following images show the effect of altering the 'Density' setting.
The density control is multiplied by this RGB value (so a blue value here means the blue light is scattered).
It is also possible to map textures to this parameter to get interesting volumetric effects. The example below uses a 3d fractal texture to simulate a cloudy fog effect.
Fractal texture mapped to Color
The attenuation parameter sets the rate at which the rays of light traveling through the scattering medium are extinguished and how much light coming from the background is blocked. A high value means that light will only travel a short distance through the volume, while a low value means that light will travel a long distance through the volume.
The attenuation control is multiplied by this RGB value (so a red value here means red light is attenuated).
Henyey-Greenstein Anisotropy coefficient between -1 (full back-scatter) and 1 (full forward-scatter). The default is 0 for an isotropic medium, which scatters the light evenly in all directions, giving a uniform effect. Positive values bias the scattering effect forwards, in the direction of the light, while negative values bias the scattering backward, toward the light. Changing the eccentricity, therefore, means that you will get a different effect depending on whether the camera is looking toward the light or away from the light.
Note that values very close to 1.0 (above 0.95) or -1.0 (below -0.95) will produce scattering that is so directional that it will not be very visible from most angles (and what scattering you do see may be noisy), so such values are not recommended.
The samples are distributed according to the volume density. More samples will refine the quality of the solution.