There are different types of yeast, just like there are different races of dogs. We call them strains, and historically they’ve been divided into two broad categories, based on how they behave during fermentation. Some of them like to swim near the top of the tank (these are called ale yeasts), and some like to swim near the bottom (these are the lager yeasts). You can guess which yeasts are used for which beers. So the yeasts are categorized based on this behaviour, but they also give different flavors to the beer (again, like dogs – they all eat the same food, but some make more hair, some make more muscle, etc – they have different metabolism).