balanced cables

What exactly do balanced cables do, and is there any difference in sound quality? What is it that gets "balanced"? I'm in the process of buying a new(used) Mac amp and preamp, and the literature says they accept balanced cables. Is this to connect the two pieces? I'd really appreciate having some light shed on this question. Thanks.
balance usually sounds better on longer runs-if your amp and pre are fully balanced, some don't really do it right. In rca ended cables, the negative is tied to groud, in balance you have a hot, negative and group. Some of the best equipment made is rca only, and some of the best is balanced. Sorry, but the answer is it depends.

On my BAT equipment, balanced definately sounds better.

Hope this helps some
The end of a balanced signal chain compares the positive signal phase to its mirror, and negative phase, and rejects anything else it may have acquired along the way as noise. This technique requires precise circuit matching and costs $$$. To take full advantage of this capability, the whole signal chain should be balanced starting with the source. However, connecting any balanced components together is helpful. You'll need balanced cables terminated with XLR connectors to hook it all up. Highly recommended! Enjoy your Macs.
Also because the negative phase is subtracted from the positive phase, not only is all unwanted signals picked up on the interconnect rejected, but the end result will have a 6db increase in gain over unbalanced.

In my case, with Krell, it makes a big difference. Krell is designed for balanced and I live near the highest TV/Radio tower west of the Mississippi. Television is a bitch, but my stereo is dead silient.

Having gone balanced I would strongly recommend any who has truly balanced gear to try it! For all the reasons the other members shared above- it is a better method of signal transfer on the right equipment.
Balanced connections have two signals, a positive and a negative signal, which are inverted versions of each other. They carry the signal currents. At the destination, these are subtracted from each other to create the signal. Any noise that is the same on both cables will be subtracted-out. There is also usually a ground that does not carry signal current - it's function is to force the grounds in the two components to the same potential. The ground conductor can be a wire or an overall shield surrounding the other two signal conductors.

Unbalanced or Single-Ended connections have a signal and a ground RETURN. Both of these carry current and the signal is driven at the source with the return current flowing back to the source on the ground conductor. The ground conductor can be a wire or a coaxial shield, depending on the design.

XLR's are superior is a number of ways. First, they are locking, so they won't pull loose. Second, they have male and female, so you can't accidently plug them in backwards. Third, they use a two signal system comprising an uninverted and an inverted signal plus a third wire ground, which carries no signal current. This "balanced" or "differential" signalling system allows the receiving component to have better noise rejection than with Single-Ended (SE) signalling. This is possible because the receiving component subtracts the + and - signals to get the signal that is used in the component. If there is any noise that is common to both + and -, this will be eliminated by the subtraction process. This is referred to as "common-mode" noise, which can include 60 Hz hum, Schott noise, RFI etc...

The thing that you need to be aware of is that all designers do not have good designs for Balanced inputs and outputs. To determine which of SE and Balanced sounds better, you need to listen to both on a given component and choose the one that sounds best.
Do stick with balanced, but here is an interesting aside. I am running my Cd player to my McIntosh 2102 tube amp, using the RCA connections. I have a tuner that also has only RCA connections so I contracted The Cable Company to make me a set of cables going from RCA to XLR. To get the pins right the technician called McIntosh and McIntosh said this was silly as the 2102 amplifier converts the signal to unbalanced so the 'cleaner' path is tto use RCA-RCA. Again, it probably would be better to run XLR-XLR, but this is an interesting aside.
Thanks to all who have provided insight with this question. The Mac literature actually minimizes the importance of balanced except for long runs.