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.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.
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.
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.