How do light dimmers change current or voltage?

[Jules18]

Some homes have light dimmers that are operated by rotating a knob. What is being changed in the circuit to create this dimming effect, and how?

I'm sure the answer is either current or voltage. Other than that I have no idea how to answer this question.

Any help is greatly appreciated.

 

[LowlyPion]

Here is a link you may find useful:
http://www.epanorama.net/documents/lights/lightdimmer.html#basics