I think you mean balanced inputs on amps (and balanced outputs on preamps).
Balancing is a way of minimizing RFI (radio frequency interference) in wires. "Unbalanced" achieves the objective by shielding wires. "Balanced" achieves the objective by sending (highly simplified explanation here) two equal but opposite versions of the signal which are then combined to eliminate any noise that creeped in during transmission.
My $0.02: Balanced is AWESOME in noisy RFI environments (like a car) or if you have very long wiring runs (like for concert venues). Note that most home use doesn't seem to fit either, so draw your own conclusion about my opinion of the value of balanced signals.
Let's say you have signal 'A'. Balanced sends two versions of the signal (A and -A) down the wire. A certain amount of noise 'X' will be induced in each run (A+X, -A+X). Thus, you can do the following at the receiving end:
Let W = -A+X
Let Y = A+X
(Y-W)/2, which results in the original signal A. Note that the divide by 2 step can be ommitted (since you'll be amplifying the signal).
The 'new' jack you refer to is called XLR and is the standard on pro audio gear (where interference over long runs are the norm).
JS, the above members are correct, XLR (balanced) design connections help kill noise. First introduced by pro users, now home audio and home theater companies have implemented these I/Os. On the same section you posted (amps, receivers, ad processors) is a thread about what prepros (audio preamp processors) have XLR connections. You can learn a lot from that thread on the subject also.
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