Fog is nothing but cloud on ground.The visibility is supposed to be less than one kilometer.Mist is thin fog.The visibility will be between one and two kilometers.
Dew is water in the form of droplets that appear on thin exposed objects in the morning or evening.Water droplets result due to the condensation of moisture at a rate greater than that of which it can evaporate.Dew does not require condensation nuclei(dust or smoke particles) for condensation whereas fog and mist require these nuclei for condensation (just like cloud).

for convenience:

Dew - Water that collects on things overnight.

Fog - A very dense form of mist