Operational amplifier
An operational amplifier or op-amp is an electronic circuit module (normally build as an integrated circuit) which has a positive (+) and a negative (-) input and one output. The output signal is the difference between the positive and the negative input amplified by the open-loop gain: output = ((inplus) - (inminus)) * gainopenloop. Since op-amps have uniform parameters and often standardized packaging as well as standard power supply needs, they help in designing an application fast: Audio pre-amps, all kinds of regulations and analogue calculators, A/Ds and D/As, ...
Open-loop gain is defined as the amplification gain from input to output without any feedback applied. Open loop gain is in theory (that means in first approximation) infimitesimally high, just as it´s frequency bandwidth is. Therefore the amplification gain for the actual application can be set very simply and exactly by using negative feedback. Of course theory and practice differ, op-amps have limits which the designer must keep in mind and sometimes workaround. The best known stumbling stone is the tendency for resonance at the upper frequency limit where negative feedback changes to positive feedback due to parasitic lowpasses.