When identifying and controlling electromagnetic interference (EMI), discussion of common-mode and differentialmode interference will likely dominate.
Differential-mode interference is a signal that appears on two lines of a closed loop, but current flow is in opposite directions. This kind of interference essentially appears in series with the desired signal. The solution is an inductor in series with the high side (and/or low side) of the line and a shunt capacitor across the lines.
A common-mode EMI signal appears on the two signal lines simultaneously in the same direction and phase. The signal on each line returns through a common ground. The solution, a common-mode choke, produces equal and opposite magnetic fields that cancel one another. A capacitor from each line to ground is also used.