This video clip shows a basic Triac dimmer wavefirm as the dimmer is turned from minimum up to full brightness, then back down.

At the 30 second mark, and again at the end, you can see the smooth sine wave voltage shape that is how power arrives at your home, and what goes to the bulb when it is fully turned on (or through a simple on-off switch).

In between you can see the truncated and somewhat tortured waveform as the Triac 'slices' each half wave.

[video:youtube]http://youtu.be/bB8EAIPAMQc[/video]

This article illustrates some of the factors at play when using and retro fitting LED lighting.

Some well presented info from Lutron

Much more technical LED dimming article