Fog is the condensation of the moisture in the air. Instead of being held in the air as vapor (or gas), water forms little droplets of liquid.

Fog forms when air cools so much that it can no longer hold water as vapor. A fog bank is an area of fog that is sharply defined. Fog banks often form at sea where cool air moves quickly over the surface of the warm ocean. The cool incoming air lowers the temperature of the air just above the water surface (which holds a lot of water from evaporation off of the ocean) and water vapor condenses into fog. Fog banks cause a lot of problems for navigation of ships.

Large areas of fog are best driven away by the sun. The sun warms the top of the fog causing evaporation. It also heats the ground surface below the fog encouraging evaporation there, too. Wind is another reason that a fog bank might break up. Hope this helps!