Starting in 2010 diesel engines have to meet stringent emission requirements.  Most diesel engine manufacturers have chosen a technology called Selective Catalytic Reduction (SCR) to meet these requirements. 

If you purchase a car, truck, tractor or any diesel powered product with SCR technology, you will need DEF.

Diesel Exhaust Fluid (DEF) is a very pure solution of urea which meets the automotive industry specification defined by ISO 22241.  It is a clear liquid similar in appearance to water.

DEF is safe to handle and use; it is not explosive, flammable, toxic, or subject to any hazardous product regulations.  It has a shelf life of 18 months if stored properly.

Since you will use less DEF than diesel, the DEF tank will be smaller than the diesel tank.  It will be clearly marked as DEF and is likely to be mounted next to or near your vehicle diesel tank.

It is important that you do not put diesel in the DEF tank or DEF in your diesel tank.  Some vehicles will have safeguards to ensure this doesn't happen but this might not be the case on all vehicles - so make sure to check!

Your vehicle will also be fitted with a DEF gauge near your fuel gauge so you will know when you need to fill up with the fluid.  If you do not fill up with DEF, your catalytic converter could damage and cause your vehicle's engine to no longer be under warranty.