Technical Corner - What is Diesel Exhaust Fluid?

Phil B here. I know the vast majority of our readers don't have a diesel powered vehicle, and those that do probably don't need Diesel Exhaust Fluid (DEF for short).  You're probably thinking that this is a joke thing that you send the new guy to fetch from the warehouse, like muffler bearings or blinker fluid.  Its a real thing though so I thought it might be interesting for the nerds among us to delve into the science of it.