EGR Systems explained.

EGR stands for exhaust gas recirculation. It is a system that injects exhaust gas back into the cylinders. This reduces the amount of combustible air (oxygen rich) in the cylinder. Less oxygen means less bang, less bang means lower temps in the cylinder. Environmetally bad Nitrogen oxide(NOx) forms at high temps. By lowering the temp in the cylinder less NOx is formed. However because of the lower combustion temps there is incomplete combustion, this causes particulates such as carbon to form. This carbon then get shot out the exhaust manifold where some gets put back into recirculation & the rest goes over the turbo & out the tailpipe (some engines have Diesel Particulate Filters (DPF) to catch this from entering the environment, but this filter, (because of its placement) is doesnt stop the carbon from wearing your turbo vanes or reentering the engine where it scours the cylinder walls).
  The exhaust is extremely hot when leaves the cylinder, the portion of exhaust going into recirculation passes over an EGR Cooler before entering by the intake manifold. This cooler is a heat exchanger (a radiator), it is supposed to cool the exhaust before it goes into the engine. However trying to cool hot gas with hot coolant achieves very little & the exhaust air entering the intake is hot. This hot air mixes with oil vapor from the rocker cover & burns. As you probably know warm oil (even oil vapor) flows freely. Burnt oil cooks into black sludge, so the EGR system cooks this oil vapor & coats the intake. This reduces how easy the engine breathes, a lot of people upgrade the exhaust (good move) but don't consider the air intake. Only air taken in can be moved out, restricted intakes greatly reduce power. Over time the sludge builds up & gets so bad it chokes the engine (hence the maccas straw blurb). The pic on the right show some build up on a std engine intake at 120,000kms. Eliminating the hot gas stops this burnt sludge. Catch cans are a popular & effective method of reducing the sludge but do not stop the carbon particulates. The picure below shows the EGR system on a 4JJ1 as found on the Holden Rodeo, RC Colorado & the Isuzu D-max.

A common way of reducing the amount of Exhaust Gas from entering the intake is by the use of a restrictor plate. In todays modern engines the EGR system can not simply be unplugged or fully blanked off due to the feedback system in the complex Engine Control Module which runs routine diagnostics. Small openings in the plates are required in order to prevent the ECU from throwing an engine light. The ECU must see a small amount of flow or it thinks the EGR valve is buggered & throws an engine light (the way around this is with an EGR Bypass Kit).  A restrictor plate for the 4JJ1 engine is shown below after 10,000kms. The carbon particulates can be seen accumulated on the plate.