What is the difference between dew and rain?
Dew is condensate that forms on the surface, usually by the surface cooling to a temperature below the dew point of the air. Thus the water vapr accumulates onto the surface.

Rain is formed by Collision-Coalescence or the Bergeron-Findelsen process. The cloud droplets are formed by the condensation of water vapor on aerosols, when the air reaches supersaturation via the Kelvin Effect and Raoult Effect (which makes up Kohler Theory).