Application note(PDF) on EMC from ST Microelectronics.
Because many electronic circuits are in proximity to each other, it is essential that their design is not affected by external noise sources and that the circuit itself is not a noise source affecting other circuits. This relationship is known as electromagnetic compatibility or EMC.
Sources of electromagnetic noise are numerous and have both natural and man-made origins. Natural sources below 10 MHz are dominated by the atmospheric noise generated by electrical storms. Above 10 MHz, natural sources consist primarily of cosmic noise and solar radiation.
Man-made noise include AM, FM, TV and other broadcast transmitters, mobiles radios but also computing devices, motors, appliances, power lines, auto ignition systems and many others. With the proliferation of these sources (particularly in urban areas), electromagnetic noise has reached important levels.