Also sometimes called staining. This is where the paint finish can be damaged or etched showing discoloured areas in the paint.
This can be caused by a multitude of different environmental factors including petrol, oils, acid, tree saps, tar, insects, farm sprays and most commonly bird poo!
Wash the surface carefully with detergent and water. Rub the area using a fine compound and then polish. In very severe cases the area may need to be sanded and refinished.