Definition of balanced line: a transmission line constisting of two conductors which need to have the same impedance, also with respect to ground. In order to establish a balanced line, every component connector to the line (incl. line driver, line or cable inself and the line receiver) must be balanced. E.g.: In a perfect world, a balanced cable consits of two identical wires which are usually twisted, surrounded with a cable shield. Balanced connections in an audio system are designed to reject both external noise, from power wiring etc, and also internal crosstalk from adjacent signal cables. The basic principle of balanced interconnection is to get the signal you want by subtraction, using a three-wire connection. Any noise voltages that appear identically on both lines (ie common-mode signals) are in theory completely cancelled by the subtraction. While two wires carry the signal, the third is the ground wire which has the dual duty of both joining the grounds of the interconnected equipment, and electrostatically screening the two signal wires by being in some way wrapped around them.
Usage of balanced lines
Mainly used for mic connections. The symmetrical construction (- phase & + phase) discriminates against noise and crosstalk, especially required in case of low voltage mic level connections. Without balanced mic lines, live gigs with lighting applications, wouldn't be possible. Even though you deal in recording or broadcast studios with shorter lines and less disturbance, balanced lines are a necessivity to achieve the high quality standards. For long runs of lines signals it is also recommended to use balanced lines instead of unbalanced instrument cables.