DPF is an acronym for Diesel Particulate Filter.
A DPF works in conjunction with the oxidation catalyst and EGR valve to remove a majority of the NOx, particulate matter (think black, cough-inducing soot) and unburned hydrocarbons from burned diesel fuel. Soot is a natural byproduct from the combustion of diesel fuel. Inside the DPF is a porous honeycomb structure that catches the soot as it passes through. After the soot builds up over time, the onboard computer controls fuel injection to allow unburned fuel to enter the filter at measured intervals where it flares off and generates increased temperatures that incinerate the accumulated soot. The result: Soot is chopped by over 90-percent.

The part usually cleans itself when the engine runs at high speeds - a process called regeneration. If this hasn't happened for a while, a warning light indicates that the filter is getting blocked. It must then be cleared by driving the car at high revs for a sustained period of time. This is easily done on the open road but is much more difficult to do around town.