Amplifier (general symbol) An amplifier circuit with one input. Really this is a block diagram symbol because it represents a circuit rather than just one component.
Light Emitting Diode A transducer which converts electrical energy to light. Usually abbreviated to LED.
Microphone A transducer which converts sound to electrical energy.
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.