There are a number of approaches to dimming a light bulb.
Simply reducing the voltage works well if you're talking about a standard light bulb.
For motors, the speed is sometimes adjusted by electronically blocking part of the 'pulse' from the AC supply.
For fluorescent lights, dimming is accomplished (in part) by changing the frequency of the AC to the bulbs.
In other words, how you "dim" something depends very much on just what that "something" is. That is why it is essential to match dimmers with the appliances.
