The dew point is the temperature that the air would have to be cooled to (at constant pressure) in order to reach saturation.
As the temperature drops to the dew point, water vapor molecules condense. If there's a solid surface present, dew may form. If no solid surface is present, cloud or fog droplets form.
Relative Humidity tells you how much water vapor is in the air compared to how much the air can "hold" at a given temperature.
The relative humidity isn't a very good measure of actual water vapor in the air because it depends strongly on temperature.
Relative humidity often changes from near 100% around sunrise to 50% or less in the afternoon, with little or no change in the actual amount of water vapor in the air. Dew point, however, gives a better reading on how much moisture is in the air.
Think of relative humidity as describing how much water is in a glass, and the glass is the air temperature. If you had a 4-oz. glass and fill it full of water, it would be 100% full. If you poured those 4 oz. of water into an 8-oz. glass, the actual amount of water would not change, but the new glass would only be 50% full. The relative humidity works the same way.