Most electrical signals are single-ended, comprised of a single wire and ground. Differential signals use two wires which are the inverse of each other -- when one swings positive, the other swings negative in equal magnitude. The receiving circuit looks only at the difference between the two, ignoring any common-mode voltage. This "push-pull" arrangement reduces the impact of electrical interference because external noise will affect both wires equally and the common-mode rejection will ignore the noise.
Examples: RS-422, RS-485, professional audio signal standards (especially for microphones), the signal lines employed by Ethernet, and the standard twisted-pair analog telephone (POTS) line.
Also see the tutorial, Understanding Common-Mode Signals.
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