with a wet pipe, water is actually injected into the expansion chamber, this helps to control the sonic wave for the tuning of the pipe, Factory Pipe designed a system where you can actually tune it your self, whereas Coffmans, R&D and Westcoast it was set from the factory.
dry pipes use a fully waterjacketed expanshion chamber that solely relies on the design of the pipe to control the tuning.
both pipes inject water into the stinger sections to keep the excaping gases cool enough to not burn out the hoses.