A potentiometer has 3 contacts and is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.
A rheostat has 2 contacts and is usually used to control current. Uses include controlling lamp brightness or motor speed and changing the rate of flow of charge into a capacitor in a timing circuit.
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.
Relay An electrically operated switch, for example a 9V battery circuit connected to the coil can switch an AC mains circuit. The rectangle represents the coil. NO = Normally Open, COM = Common, NC = Normally Closed.