Yeast is a unicellular mircoorganism. It is abe to live with or without oxygen. Throug the addition of yeast to your beer it will beginn to ferment and it also influences the beers character. Yeast is split into top-fermented and bottom-fermented yeast. Top-fermented yeast fumes during the fermention and at the end of this process  to the top. Whereas bottom-fermented yeast sinks due to the slight cell surface. The top-fermented yeast tastes much more aromatic than bottom-fermented. Top-fermented sorts are for example top-fermented dark beer, Koelsch and wheat beer. Lager beers, Pilsner, Export and Maerzen are examples for bottom-fermented beers. Louis Pasteur is a name you should realte to yeast. He proved that it is absolutely neccessary that microorganism are present for a fermentation. Some yeasts are of a negatvie natur because they are responsible for the perdition of groceries.