An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).
LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
A capacitor stores electric charge. It can be used with a resistor in a timing circuit, for smoothing a supply (it provides a reservoir of charge) and can be used as a filter (blocking DC signals but passing AC signals). Unpolarised capacitors usually have small values, less than 1µF.
Light Emitting Diode A transducer which converts electrical energy to light. Usually abbreviated to LED.