Resistors are used to limit voltage and/or current in a circuit and can have a fixed or variable resistance.  Variable resistors (often called potentiometers or rheostats) are used when dynamic control over the voltage/current in a circuit is needed, for example, in a light dimmer or volume control.

A diode allows current to pass easily in one direction and blocks current in the other direction.

Circuits containing transistors are packaged into integrated circuit chips that allow encapsulating complex circuit designs (CPU, memory, I/O) for easier integration into electronic devices and machines.

Voltage (V) is the electrical potential difference between two points. Electrons will flow as current from areas of high potential (concentration of electrons) to areas of low potential. Voltage and current are directly proportional in that the higher the voltage applied to a conductor the higher the current that will result.

Connecting the 6 batteries in series multiplies their voltage while keeping their current the same yielding a 54V 15A configuration. Connecting the 6 batteries in parallel multiplies their current while keeping their voltage the same yieleding a 9V 90A configuration. Using a series-parallel connection, 3 batteries can be connected in series and 3 can be connected in parallel resulting in a 27V 45A configuration.