What’s the difference between top and bottom fermenting yeast?
It’s pretty simple really. Top fermenting yeasts do not ferment at the top, and bottom fermenting yeasts do not ferment at the bottom.
All varieties of yeast fully permeate the wort during fermentation. “Top fermenting” yeasts are called so because they tend to generate a thick foamy layer along the top, contributing to krausen. “Bottom fermenting” yeasts, most often used for lagers, do not contribute as much this foam layer. All yeast will settle to the bottom of the fermenter leaving a sediment.