Electromagnetic Interference (EMI) is often used interchangeably with Radio-Frequency-Interference (RFI). Technically, EMI refers to the type of energy (electromagnetic), while RFI refers to the frequency range of the noise frequency. Both EMI filters and RFI filters describe unwanted signals (noise) that EMI filters and RFI filters are intended to eliminate. EMI filters and RFI filters consist of a circuit or device containing series inductive (load-bearing) and parallel capacitive (nonload-bearing) components, which provides a low impedance path around the protected circuit for high frequency noise. Filters also attenuate impulses since a Fourier's Analysis of a spike will reveal it is composed of high- frequency waveforms. Filters and surge suppressors when used together thus act synergistically.

EMI filters and RFI filters can be single phase, three-phase, or DC.  Single phase is a type of alternating current (AC) with a single output which may be center tapped for dual voltage levels.  Three-phase is a type of alternating current (AC) with a single output that may be center tapped for dual voltage levels.  DC or direct current (DC) is one that flows always in the same direction.  The package type for EMI filters and RFI filters can be module, desktop, or stand-alone.

The most important specifications for EMI filters and RFI filters include rated voltage, rated current, and insertion loss.  The rated voltage is the voltage for which the EMI / RFI filter was designed.  The rated current is the current for which the EMI / RFI filter was designed.  Insertion loss is a statement of the filter's attenuation characteristics, exercising in decibels (dB) the ratio of noise that would get through without the filter, to that which gets through with the filter installed.  Another important specification to consider when searching for EMI filters and RFI filters is the operating temperature; the temperature range the filter was designed to operate correctly.

Important approvals for EMI filters and RFI filters include CSA mark, UL listing mark, UL recognized component mark, VDE component mark, and Mil Spec.  The CSA Mark may appear alone or with indicators. If it appears alone, it means that the filter is certified for the Canadian market, to the applicable Canadian standards. If this Mark appears with the indicator "C and US" or "NRTL/C" it means that the filter is certified for both the U.S. and Canadian markets, to the applicable U.S. and Canadian standards.  The UL listing mark denotes that Underwriters Laboratories Inc. (UL) has found that samples of the filter met UL's safety requirements. These requirements are primarily based on UL's published Standards for Safety.  UL recognized component marks are used only on component parts that are part of a larger product or system. These components may have restrictions on their performance or may be incomplete in construction. Products intended for Canada carry the Recognized Component Mark "C."  VDE component marks are for electronic components that meet Verband Deutscher Electrotechniker (VDE) standards.  Mil-Spec certifications are awarded to filters that meet military specifications.