1. What is EMI / RFI shielding?

EMI/RFI stand for ElectroMagnetic Interference and Radio Frequency Interference (RFI is just EMI over the range of 1kHz - 10GHz). Electromagnetic radiation and radio frequency radiation are all around you. Their effects on the human body are the subject of fierce debate, but their effect on electronic devices is clear. They interfere. Sensitive components and circuits can be disrupted by interference from sources like power supplies, fluorescent lights, electric motors, switches and relays, even IC's. The clock in your computer oscillates and produces interference. In short, almost all electronic circuits develop some level of EMI and / or RFI.

Shielding from EMI / RFI is done for two primary reasons: (1) to keep your electronic device from interfering with others, or (2) to keep other devices from interfering with yours. Often, regulatory agencies such as the U.S. FCC or EU dictate standards for both emissivity of EMI/RFI and sensitivity to interference.