Yeasts are of two kinds. One ferments at the top of the tank at a higher temperature for a shorter time, producing fruity characters. The other ferments at the bottom, but requires a lower temperature and a longer fermentation time. It produces more neutral characters. The two basic styles of beers are ales and lagers.
Lager yeast is very delicate yeast.
It takes longer to brew and likes to be fermented at a lower temperature. Ale yeast, on the other hand, is very forgiving. It can cover up fermentation flaws because it has such a powerful flavor itself. The type of hops used is another element that differentiate ales and lagers. American beers are lagers.