Vapor Pressure Deficit (VPD) is the difference in vapor pressure within a plant's leaves and the surrounding air.
VPD is useful because water and nutrient uptake, carbon dioxide absorption and other processes critical to plant growth and development are driven by transpiration, which is driven by this vapor pressure deficit. If VPD is too low, transpiration slows along with plant growth and other problems like mold can arise. If VPD is too high, excessive transpiration can trigger the plant to take defensive actions which can cause other issues such as leaf tip burn.
Dewpoint is the temperature at which the air will become fully saturated with water vapor.
Relative humidity is the percentage of the air's water vapor capacity that is currently occupied. Since the amount of water vapor that air can hold increases with warmer temperatures, air with the same amount of moisture will report different relative humidity readings at different temperatures.
If the air with a constant amount of moisture is warmed, its vapor capacity increases and thus, the percentage filled, or relative humidity, will decrease. Conversely, if the same air is cooled, its vapor capacity decreases and the relative humidity will increase. The dewpoint is equal to the temperature at which the relative humidity would reach 100%, indicating that the air is full. At this point, the vapor will begin to condense, or change into a liquid state and appear as fog, mist, dew, frost, etc. depending upon the specific environment.
Dewpoint can be useful as a measure of comfort as well as a means of comparing the absolute amount of moisture in two volumes of air, such as when making decisions about ventilating a humid space. Depending upon the specific values, warmer air with a lower relative humidity can actually have more moisture than cool damp air, making ventilation counter-productive in attempts to reduce humidity. Comparing dewpoints (lower dewpoint means less water vapor) can better inform these decisions.