A NOR gate can have two or more inputs. The o on the output means not showing that it is a Not OR gate. The output of a NOR gate is true when none of its inputs are true.
This type of variable capacitor is designed to be set when a circuit is made and then left without further adjustment.
Ohmmeter Measures resistance. Most multimeters have an ohmmeter setting.
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.