Breaking down what yeast eats?

[blazingsun81]

We all hear that yeast will break down waste material to clean up our brews.
I was curious what this waste material is

 

[Yooper]

I think most often what is being referred to is diacetyl in that case.

http://www.professorbeer.com/articles/diacetyl.html has a simplified write up on the ways diacetyl is created, and on ways to get rid of it.