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.
An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
Phototransistor A light-sensitive transistor.
Piezo Transducer A transducer which converts electrical energy to sound