Humidity is the measure of the amount of water vapor in the air. Warm air can hold more water vapor than cool air. Air that is saturated has as much water vapor that it can hold. Measure by grams of water that are found in a cubic meter of air (g/m³). High numbers reflect more water vapor in the air. Low numbers reflect less water vapor in the air.
Relative Humidity is shown as a percentage. It is a measure of the amount of water vapor in the air as a percentage of the maximum amount that saturated air can hold at a given temperature.
When the air reaches 100% relative humidity, the air is saturated and cannot hold anymore water vapor. Rain or dew will form. The dew point temperature is the temperature at which the air is saturated.