What is the difference between balanced and unbalanced audio inputs?

A balanced audio connection has two wires, one of inverted polarity to the other. (For instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an upside-down version of the same signal.) However, an XLR plug also carries a third connection - pin 1 is used as an earth to shield the other two. The received signal is the difference between the two signal lines. This signal recombination can be implemented with a differential amplifier where the negated signal is tied to the negative terminal of the operational amplifier. A balun may also be used instead of an active differential amplifier device. Much of the noise induced in the cable is induced equally in both signal lines, so this noise can be easily rejected - the noise received in the second, negated line is applied against the first, upright signal, and cancelled out when the two signals are added together.