HDD stands for Heating Degree Day and is "the number of degrees that a day's average temperature is below 65o Fahrenheit (18o Celsius), the temperature below which buildings need to be heated."
CDD stands for Cooling Degree Day and is "the number of degrees that a day's average temperature is above 65o Fahrenheit and people start to use air conditioning to cool their buildings."
GDD stands for Growing Degree Day and is "a measurement of the growth and development of plants and insects during the growing season. Development does not occur at this time unless the temperature is above a minimum threshold value (base temperature)." Weather Trends uses a base temperature of 50o F (10o C).