Looking at the left hand diagram, an input with a swing of a fraction of a millivolt produces an output that changes between + 12 volts and - 12 volts.

In most cases this gain is excessive, and is reduced by negative feed back.

Looking at the right hand diagram we can see that the opamp amplifies right down to dc.

Gain falls quite rapidly as the frequency increases.

In fact the bandwidth (the point at which the output has fallen by 3 dB) is only 1 kHz.

This is also improved upon by the use of negative feedback.

The input impedance is high, 1M.

The output impedance is low, 150 ohms.