The circuit shown below is called an inverting amplifier.










Input voltage V1 is applied to the inverting input terminal, which may be a DC or AC voltage. R0 is the feedback resistor. As non-inverting input terminal is grounded, inverting input terminal is at virtual ground (Va=Vb=0).

The current flowing through the resistor R1 is given by

                        I1 =  V1- Vb  =  V1
                                R1            R1

(as Va=Vb=0)

The current flowing through R0 is given by
                         I0 = Vb- V0  =  - V0
                                 R0            R0

Since the input impedence of opamp is infinite and the input currents are zero, I1=I0. Therefore from equation (a) and (b), we get
V1 = -V0
R1     R0
         
Therefore,
V0 = R0  x V1 
       R1

The gain of the amplifier is R0/R1.

Summary
When opamp is used as inverting amplifier with positive voltage going into inverting terminal, the resultant output is negative voltage. In other words, signal reverses polarity (inverts polarity).

Two resistors are needed to make the 741 work as an amplifier, R1 and R2, where R0 is used as feedback resistor.