Nearly all LEDs operate from low voltages and require a Power Supply Unit to reduce the mains voltage supply down to the voltage or current that is required by the LED product. PSUs are either constant voltage or constant current (See FAQs).
LED drivers are electrical devices that regulate the power to an LED or string(s) of LEDs.
Different from conventional power supplies
What makes a driver different from conventional power supplies is that an LED driver responds to the ever changing needs of the LED(s) by supplying a constant amount of power to the LED as its electrical properties change with temperature. Without the proper driver, the LED may become too hot, become unstable resulting in performance issues or most likely have a cataclysmic failure.