Using two wires for each electrical path for high immunity to noise and crosstalk. The signals are sent down one wire as positive and the other as negative, and the circuit at the receiving end derives the signal from the difference between the two.
Differential Vs. Single-Ended
Differential signaling is superior to single-ended signaling, where the reference voltage is ground and can vary over long distances. In differential signaling, the reference voltage is controlled. Typically, twisted pairs are used so that external radiation impacts both wires at the same place, and the noise can be detected more easily. See LVDS