An LDR, or Light Dependent Resistor, is a type of resistor that changes its resistance based on the amount of light it receives. In bright light, the resistance decreases, allowing more current to flow through, while in darkness, the resistance increases, reducing the current. This property makes LDRs useful in various applications, such as light sensors and automatic lighting systems. LDRs are commonly made from materials like cadmium sulfide, which are sensitive to light. They are often used in devices like streetlights, which turn on automatically at dusk, and in photographic equipment to adjust exposure settings based on lighting conditions.