There are many different strains of brewer's yeast available nowadays and each strain produces a different flavor profile. Some Belgian strains produce fruity esters that smell like bananas and cherries, some German strains produce phenols that smell strongly of cloves. Those two examples are rather special, most yeasts are not that dominating. But it illustrates how much the choice of yeast can determine the taste of the beer. In fact, one of the main differences between different beer styles is the strain of yeast that is used.
Most major breweries generally have their own strain of yeast. These yeast strains have evolved with the style of beer being made, particularly if that brewery was a founder of a style, such as Anchor Steam. In fact, yeast readily adapts and evolves to specific brewery conditions, so two breweries producing the same style of beer with the same yeast strain will actually have different yeast cultivars that produce unique beers. Several yeast companies have collected different yeasts from around the world and offer them to home brewers. Some homebrew supply shops have done the same, offering their own brands of many different yeasts.