Light Emitting Diode A transducer which converts electrical energy to light. Usually abbreviated to LED.
Preset variable resistor A preset is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than standard variable resistors so they are sometimes used in projects to reduce the cost.
Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the page on logic gates. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).
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).