EGR stands for Exhaust Gas Recirculation. This system is on nearly every vehicle as it helps manufacturers meet emission standards required. This system does as the name says, it recirculates exhaust gas back into the engines intake to be re-burnt. This does 2 things, it increases the fuel economy and reduces pollution.
This system does however cause a lot of problems mainly on diesel engines, more so on higher mileage vehicles. As diesel engines produce a lot more carbon ( soot ) the EGR valve can block up or seize. Also I have also seen intake manifolds with large amount of soot/carbon build up restricting the air inlet.
Today was a great example, this Holden Colorado was blowing excessive smoke and not driving as it was in limp mode (safety mode set by the computer) the cause was the EGR valve was seized in the open position. It was replaced with a new unit and the fault codes cleared from the computer and the vehicle was good to go. You can see in the pic below the valve in the stuck open position.
The 2nd picture is from another breakdown. A BMW X3 which had the engine light on the dash. This was the same fault as the Holden, the EGR valve had failed. This had to be replaced to rectify the fault. Once replaced and tested the vehicle was running 100% again.
So if your vehicle has its engine light on, or suspect an EGR problem call your local mobile mechanic.