Dew-Point

Simply put, the dew-point is the amount of moisture in the air.  It is indicated by a temperature at which dew forms.  According to the University of Illinois, dew-point is the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation.  The dew-point is never higher than the air temperature.  When the dew-point temperature is at the air temperature, the air is said to be saturated.

When the dew point is at higher temperatures there is a greater likelihood of clouds and storms forming. There is also a higher likelihood that cooling could occur, causing water vapor to condense, and form dew.

Relative Humidity

Relative Humidity is indicated by the difference between the air temperature and the dew-point temperature.  It is the amount of moisture in the air determined by what the air has the potential for.  If the difference between the temperature and the dew-point is great, the relative humidity is low (10%).  If these numbers are very close together the relative humidity is very high (90%).