Voltage is the electric potential in a circuit that gives potential for current to flow. However, it doesn't mean that the circuit has current while voltage is present in a circuit. Voltage is the force that pushes electrons through the circuit when the path is closed for current to flow. Current is only present when there is a closed, completed path for current to exist.
Current is the rate at which electrons flow past a point in a complete electrical circuit.
Frequency is typically used to describe electrical equipment operation. It is the rate at which current changes direction per second.
Load regulation is the relative difference of output voltage at no load and full load.
Efficiency is the ratio of the output power to the input power pin. [ Pout / Pin * 100% ]