aka “Saccharomyces pastorianus”, “S. pastorianus”, “Saccharomyces carlsbergensis” or “Saccharomyces monacensis”, “bottom-fermenting yeast”.
Saccharomyces pastorianu named in honour of Louis Pasteur, is the yeast that we commonly refer to as “bottom-fermenting” lager yeast.
Genetically ale yeasts and lager yeasts are very closely related. So closely in fact that they are often considered to be members of the same family, pastorianus being a member of the wider cerevisiae family.
Lager yeast generally likes to ferment at lower temperatures than ale yeast, producing producing cleaner flavors. If fermented too hot lager yeast tend to product more off-putting phenolic flavors than ale yeast.