What is dew and how is it different from frost?

Air has a certain amount of moisture in it at any given time. Warm air can hold a lot more moisture than cool air. On a summer day with moderate humidity, we don't notice the moisture in the air. At night, when the air cools down considerably, the cooler air can't hold all of the moisture, so some of it precipitates out of the air onto whatever surfaces the air surrounds. That's dew. If it becomes a very cold night, the dew freezes. That's frost.