To be honnest I only just heard of them the other day watching 10 greatest diggers or something

'Aftercooler
The aftercooler cools the compressed air from the pump before it enters the receiver. The heat from the compressed air is transferred outward to the aluminum fins of the aftercooler, the flywheel/fan creates an air flow that rushes past and cools the aluminum fins.'
http://www.generatorjoe.net/html/glossary.htm
I gather they have a waterpump (or at least the ones I found info on), but how is this different to an w/a intercooler?