Inverting amplifiers are negative feedback op-amp amplifiers in which the input signal is applied to the inverting input pin itself along with the feedback system on the same pin. Usually the feedback system will be a simple voltage divider using two resistors.
You might have noticed that there is a negative symbol in the above gain equation. The reason is nothing but the output will be negative since the non-inverting pin is grounded and we are applying a voltage on the inverting pin. In case of input signals other than DC voltages, the output wave will be having a 180 degree phase shift with respect to the waves.