An instrumentation amplifier is a type of differential amplifier that has been outfitted with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment. The ideal common-mode gain of an instrumentation amplifier is zero, which means that it is a very good at rejecting common mode voltages (typically noise from various sources including ground loops).
Although the instrumentation amplifier is usually shown schematically identical to a standard op-amp, the electronic instrumentation amp is almost always internally composed of 3 op-amps. The three-op-amp instrumentation amplifier is seemingly a simple configuration in that it uses a basic, decades-old operational amplifier to gain a differential input signal. Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are also available in integrated circuit form from several manufacturers.
EDN Magazine article Understanding CMR and instrumentation amplifiers tells that from the CMR (common-mode rejection) perspective, instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. his situation is not so mysterious when you think about the inside of this device. And the inside operation of the instrumentation amplifier is well presented on this article.