Power factor (PF) is the ratio of the real power flowing into an LED to the apparent power flowing in the circuit. It is an expression for energy efficiency and it ranges from 0 to 1.

The real power is the actual value of power required by the LED to function and apparent power is the power flowing in the circuit which is the product of the current flowing and the voltage in the circuit. 

LED drivers are the devices that are used to provide the required power to LEDs, i.e. they are electrical components that convert the line voltage to the low DC voltage needed for the LEDs. They may be either constant current or constant voltage. They protect LEDs against current and voltage fluctuations and thus regulating the power flowing into an LED.

Constant current LED drivers are those which regulate the power by varying only the voltages in the circuit, keeping the current constant and constant voltage LED drivers are designed to operate for a single DC output voltage.