Relative Humidity or 'RH' is defined as the amount of water vapour held in the air compared to the amount that the air can hold at any given temperature. The higher the temperature, the more water vapour can be held in a given volume of air. Therefore the %RH is the ratio of water vapour present into the air relative to the amount required to reach saturation or for condensation to begin to form at the current temperature.
Dew Point Calculation
The dew point is the temperature at which a sample of moist air at constant pressure reaches water vapour saturation.
Wet Bulb Calculation
The wet bulb measurement is the measurement of the saturation of the water vapour in the atmosphere being measured i.e. how much water vapour is present in the atmosphere.
So where dew-point is the determination of the point at which dew is formed, the wet bulb is the further determination of the concentration of vapour present. Both are measurements of moisture but both determine different values.