Complex sentence: A personal defense weapon (PDW) is a class of compact select-fire firearm-essentially a hybrid between a submachine gun and a carbine, retaining the compact size and ammunition capacity of the former while adding the power, accuracy and penetration of the latter.
Split into simpler sentences: A personal defense weapon (PDW) is a compact select-fire firearm similar in most respects to a submachine gun. The class of weapon as it exists today evolved as a hybrid between a submachine gun and a carbine, retaining the compact size and ammunition capacity of the former while adding the power, accuracy and penetration of the latter.