This uses closed-loop negative feedback to control the speed of a DC motor under changing load conditions.
Under closed-loop negative feedback, the voltage at the opamp V+ terminal (Vset) is what you would like to measure at the V- terminal (Vsense). The two controlled sources are a hack-job speed sensor with a gain of 5 mV/rpm.
The difference between Vset and Vsense is the error of the system represented as a voltage (error = what you want - what you have). The opamp multiplies this error by the open-loop gain: Vout = Ao*(Vset - Vsense). Our job as designers is to use this amplified error as a means of controlling our system to reduce the error. In this case, the opamp applies more voltage to the motor, causing it to spin faster. When the motor load increases, it would normally slow down, but the correspondiong drop in Vsense causes the opamp to compensate, stabilizing the system. That's the hallmark of closed-loop negative feedback: The system rejects disturbances and stabilizes to a given set point.
