Enable Smoke
Decides whether the smoke density of the volume will be rendered

If Smoke is disabled and the Fire tab is enabled, this operation replaces the density field with the fire volume field specified in the Bindings tab instead of the smoke volume field.  

If both smoke and fire are disabled then the result defaults to the input volume as seen in the image above. 
Note: this does not affect data generated from Scatter

Density Scale
Multiplies the density of the smoke volume by this value. Thicker density can produce darker colors and more opaque shadows in renders.

The above images demonstrate Smoke Density values of 20 / 50 / 100 / 200

Smoke Color
Defines the color of the smoke.