What actually is a yeast cake? I'm guessing that it's the fallout in the bottom of the fermenter, but that includes other stuff/gunk. Can it be reused?
Yes it refers to the yeast fallout in the fermenter. Yes it can be reused. Yes it does include other stuff/gunk, especially if it's from a primary fermenter. It's easier to use the yeast cake for a new fermentation, because there's less gunk, but you don't want to take yeast from a secondary past about 3 generations...evolution in place.