This circuit is for a simple buzzer that is activated by light, i.e., the buzzer is on when there's ample light and off when it is dark.  A general-purpose operational amplifier, the 741, is used as a comparator that determines whether there's enough light to turn on a self-oscillating piezoelectric buzzer. Its non-inverting input is connected to a photoresistor, a component whose resistance decreases as more light shines on it.  Its inverting input, on the other hand, is connected to an almost fixed voltage, i.e., a proportion of the supply voltage as set by trimmer resistor R2.

If there isn't enough light shining on the photoresistor, the buzzer is quiet. As more light shines on the photoresistor, its resistance decreases and causes the voltage across R1 to increase.  At a certain level of lighting, the voltage across R1, which is also the voltage at the non-inverting input of the 741, becomes larger than the voltage at the inverting input. At this point, the 741 is triggered to output a 'high' level, turning on Q1.  Q1 then activates the self-oscillating piezoelectric buzzer. 

See also:  Operational Amplifier

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