The Op-Amp is nothing more than a differential amplifier that
amplifies the difference between two inputs. One input has a positive effect on
the output signal, the other input has a negative effect on the output.
The theoretically perfect Op-Amp has an infinite voltage gain, an infinite
bandwidth and infinite input impedances. In this way it just senses an input
voltage level without actually interfering with that voltage in any way. The
perfect Op-Amp also has a zero-Ohm output impedance. It may therefore be used
to drive heavy (in electronic terms) circuits.
The high gain does come as an advantage. If a sample of the output fed back to cancel out some of the input, the effective gain is reduced, but the gain becomes much more stable. This is called negative feedback and the more gain you have to start off with, the more stable you can make the effective gain.
The precision and flexibility of the operational amplifier is a direct result of the use of negative feedback. Generally speaking, amplifiers employing feedback will have superior operating characteristics at a sacrifice of gain.
With enough feedback, the closed loop amplifier characteristics become a function of the feedback elements. In the typical feedback circuit, the feedback elements are two resistors. The precision of the “closed loop” gain is set by the ratio of the two resistors and is practically independent of the “open loop” amplifier. Thus, amplification to almost any degree of precision can be achieved with ease.