An LED driver is a self-contained power supply that has outputs matched to the electrical characteristics of your LED or array of LEDs. There are currently no industry standards, so understanding the electrical characteristics of your LED or array is critical in selecting or designing a driver circuit. Drivers should be current-regulated (deliver a consistent current over a range of load voltages). Drivers may also offer dimming by means of pulse width modulation (PWM) circuits. Drivers may have more than one channel for separate control of different LEDs or arrays.
definition from led center

Mean Well's  Cost Effective PLP-60 PCB Type LED Power Supply.
Mean Well’s Cost Effective PLP-60 PCB Type LED Power Supply. A nice built-in design for low cost applications.

LED Drivers should not be looked at as inefficient devices. Here is an example of a press release for a 96% efficiency device.

LEDs without drivers?!?! YES! Acriche™ 4W AC LED from Seoul Semiconductor can be directly plugged into the AC line voltage without a converter or separate power supply. (Watch your fingers!)

So as you can see an LED driver is the type of device that comes in many flavors. It depends on your system wattage needs, for the associated cost increase. In an attempt to recycle reuse and repurpose we, here at EcoBrooklyn, decided in a ‘build it forward’ , commonly associated with a ‘green builder’, attitude, to find some drivers of our own. With some ingenuity and help from our friends (our thanks to you) I undertook the idea of using computer ATX power supplies as a regulated Voltage source. Since an LED is a current driven device any fluctuation in voltage even down to a tenth of a volt can cause a large increase in current flow. This can damage or destroy the LED at worst or cause flickering at best, which can have quite a discomforting effect. So with this in mind and the knowledge that computer electronics are very sensitive to voltage fluctuations on a small but reasonably similar level, I had a great match up. Other standard functions include short circuit, over load, over voltage protections and over temperature shutoff. As well as more than one voltage source in the original package. This we can use to allow more than one type of LED being powered by such VCCS (Voltage controlled current source) supplies.

ATX Power Supply