An aerosol is a colloid of fine solid particles or liquid droplets, in the air or another gas. Smoke is artificial aerosol and fog is a natural aerosol. Smoke is a mixture of gases. Fog is a mixture of liquid and gases. In both fog and smoke, the dispersion medium is same, i.e. gas or air. The only difference is that the dispersed phase in fog is liquid (water particles) and in smoke, it is a solid (carbon particles).