In audio, as a means of transferring weak signals (between components, for example), there are two systems used, each of them also with its own connectors' standard.
Balanced (Symmetrical) Signal
This type of signal transfer uses three connections per channel: ground, “hot” and “cold”. The “hot” signal is identical to the“cold”, except that the second one is inverted compared to the first one. The amplifier (or any other component) receiving this signal then assembles it by summing it after the “cold” has been inverted and therefore equal to the “hot”: if the original signals have amplitude A each (“hot” and “cold”), the sum will be 2A.
Why is this system OK? If any glitch appears on the way of transfer (on the cable, for example) it will be neutralized. The same glitch appears on both signals equally: on “hot” and on “cold”. When the “cold” signal gets inverted, the glitch on it gets inverted, too. If we sum glitch and inverted glitch, we neutralize it.
Balanced signal transfers are therefore great when using very long cables (stage, studio, …), weak signals (microphone) or both. Many hi-end components use balanced inputs/outputs. The most common standard for connectors is XLR.
Unbalanced (Asymmetrical) Signal
The unbalanced transfer method uses two connections per channel only: ground and “hot”. They are easier to manage, but also more vulnerable to glitches, therefore short connections are preferred. The most common connectors' standard is RCA, also known as Chinch. Sometimes also a jack can be used (mono or stereo, 3.5 mm or 6.5 mm).
Converting Unbalanced to Balanced and vice-versa
When having one component with balanced outputs (or inputs) and the other one with unbalanced ones, we need to convert them properly. There are two ways of doing so: passive (with a transformer (left photo)) and active (with electronics (right photo)). In any case, this type of conversion always brings some distortions, noises, signal reshaping, etc. It is strongly recommended to use the components with only balanced or only unbalanced signal transfer. Mixing them will bring out some compromises.