It is an expectation that when you buy a LED light fitting you will be able to control the brightness via the same dimmer you use with your halogen or other incandescent lights. LEDs use a different method to perform dimming known as Pulse Width Modulation.
On the other hand, halogens/incandescent lights use a TRIAC dimmer (the normal dimmer you have on your wall today) which is incompatible with the LED's PWM. To avoid having to replace all your dimmers or worse still, add additional wiring, there needs to be a way to convert TRIAC dimming to PWM.
First it may be useful to provide an overview of how a TRIAC (Triode for Alternating Current) dimmer works. The TRIAC dimmer controls the amount of power supplied to the light, the lower the power, the dimmer the filament will burn. The TRIAC is very clever in that it can manipulate the AC current (which is a sine wave) by rapidly turning the power on and off at different parts of the wave. In Australia the AC current runs at 50 cycles per second (50Hz) and the TRIAC can flip flop between ON and OFF 100 times a second (2 x 50Hz). Varying the duration of the ON/OFF cycle controls the amount of power that gets to the light which in turns affects the filament brightness. For example, the longer the TRIAC is in the ON state per cycle the brighter the light.
The the following diagram shows as the TRIAC dimmer in various positions and its affects on the AC current and the brightness of the lamp.
Since LEDs are solid state devices, there is no filament that glows. LEDs can either be ON at full brightness or OFF, there is no in between. This is where Pulse Width modulation (PWM) comes in. With PWM dimming is achieved by turning the LEDs on and off at very high frequency (many 1000s of times per second) at speeds so fast that the human eye does not notice the strobing effect. The longer the on periods are relative to the off periods, the brighter the LEDs is perceived by the eye.
All LED lights have a driver (Halogens use transformers) that provides a constant current to light up the LED or group of LEDs (LEDs are current driven). The driver takes the 240/120v AC input and converts it into a DC current. It is the LED driver that must receive the TRIAC dimmed AC wave form and convert it in to a PWM signal for the LED to dim properly (turn on/off really fast). The following diagram is for illustrative purposes, in actual fact for every 100 ON/OFF cycles of the TRIAC signal the PWM signal will probably flip/flop many thousands of times. This is necessary to avoid any perceivable flicker.