In electronics, an amplifier is a circuit that multiplies an input signal by a factor called the Gain. So, if the Gain is 10, the output signal will be 10 x larger than the input signal. It is often the case that the gain is negative; in this case the output signal is still 10x bigger than the input signal, however the signal is inverted. This means that where the input signal is high, the output signal is low, and vice-versa.
One example is a single transistor. For simplicity, lets consider the digital case where we consider the transistor to be a simple on/off switch. An NPN transistor has three three terminals: the collecter, base, and emitter. If the base has a voltage on it, the switch is closed, and current flows. If the base voltage is zero, then the switch is open, and no current flows.
Now, consider the following circuit:
5V o------\/\/\/\/\/-----------SWITCH-----o GND
Take the output here^
So what is the output of this circuit based on the input to the NPN transistor? If there is no current, then the voltage drop across the resistor is 0 V, therefore the output voltage is 5V. If the switch is closed (say with a control voltage of 1 V), then the output is connected to GND, so the output is close to 0 V.
Input = 0 V --> output = 5 V
input = 1 V --> output = 0 V
The outputs here are inverted. Similarly with this switch example, if the transistor is biased to be on the verge of turning on or off (three more resistors needed), then a very small ac signal -- like an audio signal -- can be superimposed on the dc bias point and you get a true amplifier. The gain of this amplifier is still negative, for the same reasons in the example above.
An inverting amplifier is a type of electric circuit that reverses the flow of current passing through it.