Relative humidity is defined as the amount of atmospheric moisture present, divided by the amount of moisture that would be present if the air were completely saturated. This ratio is typically expressed as a percentage. Since the amount of moisture the atmosphere can hold is dependent on temperature, relative humidity is affected by both moisture content and temperature.
Meteorologists use historic data to determine the maximum amount of moisture the air holds at a given temperature. Dew point is a measure of the amount of water vapor actually in the air. If a sample of air was cooled, the temperature at which moisture would leave the air and begin to condense, is the dew point temperature. Relative humidity is the ratio of these two moisture amounts.
Relative humidity directly affects human comfort. The human body uses the evaporation of perspiration to cool itself. The faster perspiration evaporates, the faster the body cools. If the atmosphere has a high relative humidity, that means it is already saturated with moisture and perspiration will not evaporate. The body then produces more perspiration in an effort to regulate temperature. This is why people often become coated with sweat when performing physical activities on a day with high relative humidity.