Designing for board level electromagnetic compatibility application note (PDF!) from NXP:
Electromagnetic interference (EMI) is a major problem in modern electronic circuits. To overcome the interference, the designer has to either remove the source of the interference, or protect the circuit being affected. The ultimate goal is to have the circuit board operating as intended — to achieve electromagnetic compatibility (EMC).
Achieving board level EMC may not be enough. Although the circuit may be working at the board level, but it may be radiating noise to other parts of the system, causing problems at the system level.
Furthermore, EMC at the system or equipment level may have to satisfy certain emission standards, so that the equipment does not affect other equipment or appliances.
Many developed countries have strict EMC standards on electrical equipment and appliances; to meet these, the designer will have to think about EMI suppression — starting from the board level.