You can turn unbalanced into balanced in two ways, using a center tapped transformer or by using active circuits to invert the signal to one of the lines.
Electrical noise is not reduced in a balanced line but interference from outside the cable may be reduced or removed completely. The idea is that instead of using ground and one signal wire, you use ground and two signal wires. The signal wires carry identical but opposite polarity signals, as the waveform on one moves more positive, the other moves more negative and vice versa. As long as the wires are kept very close together, they are often twisted around each other, they should pick up an identical amount of interference from outside, The interference may be magnetic or electrical, what matters is that both signal wires pick up the same amount. At the receiving end of the cable, again either a transformer or differential amplifer is used to return the signal to unbalanced state. Lets call the signal 'A', so one wire carries 'A' and the other carries '-A', if the '-A' is inverted and added to 'A' the result is 2A, twice the wanted signal. Now look at the interference, call it 'B', it is the same on both wires because of their close proximity. When the invert and add circuit sees this it gives out 'B' + '-B' which is zero. So the wanted signal is doubled and the unwanted one is cancelled out.
Although balanced lines are often screened to give additional protection, it isn't always necessary to do so. It is quite common to see television signals sent hundreds of metres along CAT-5 network cable which is unscreened and there is almost no interference pick up.
Brian.