In what way are Tropical Wet and Dry (aw) climates distinguished from Tropical Wet (af) climates?

The Koppen Climate classification system is the most utilized tool for differentiating climate zones in different areas of the world.  It classifies climates into five groups, A-D.  There are three classifications in A.  The A group are areas that are very warm (above 18 degrees Celsius) and receive a lot of annual rainfall (over 1500 mm.)  The major distinction made between AF and AW zones is the amount of precipitation on an annual basis.  The Tropical Wet (AF) zone has constant rainfall because of the humidity and high temperatures during the day.  These areas are called rainforests and average over 1500 millimeters of precipitation.    The Tropical Wet and Dry (AW) zone has a very rainy season during the summer but then a dry season in winter months.  High-temperature ranges are much larger in the Tropical Wet and Dry zone which results in a dry season.  So the major distinction between the two zones is the dry season that exists in AW.  Tropical Wet and Dry zones tend to exist on the fringes of Tropical Wet zones.