It is basically a variable resistor whose resistance varies according to the light intensity falling on it.
The resistance of the LDR can be few Mega Ohm’s (MΩ) in dark conditions, and in Light condition the resistance can be of several hundred ohm’s.
LDR’s are less sensitive to lights as compared to photo-transistors or Photo-diodes.
Photo-transistors or Photo-diodes are active components with P-N Junction whereas the LDR is a passive component. The ZigZag Pattern of the LDR on the ceramic Substrate increases the dark resistance.
LDR has no polarity just like a resistor. The resistance of LDR can be very easily measured by just connecting the two terminals of LDR to a multimeter & set the multimeter on resistance Mode. Now when you change the intensity of light falling on the LDR you can see the resistance varying.