Differential amplifiers are completely balanced amplifiers of high sensitivity with two inputs and one output. The inputs are one non-inverted and one inverted input. The inputs have identical gain factors and equal frequency responses. The amplified differential signal is available at the output.
The input signals are at the inverting and the non-inverting input. If they are identical in the input level, then the output level is zero. Only if there is a difference between the input levels will a differential voltage build up between the outputs. So the output amplitude is always equal to the amplified difference between the input levels.
Identical input signals are suppressed, which is expressed as a high common mode rejection. This property is also used for noise suppression. If interference signals with the same polarity reach the two inputs, they are compensated by the common-mode suppression. This applies equally to hum signals as well as to interspersed RF signals.