well, in a nutshell, yeast is different in different environments. so, you have yeasts that have undergone years of selection to produce the right profile for different styles. do you mean like bread yeast as "regular" yeast? it's just a different strain of yeast that has slightly different characteristics. think of it like different cultivars of a plant. bell peppers for instance. some stay green, some turn orange, some turn brown, some red, some are large, some small, etc... similarly, some yeast strains produce more ester and phenolic character, like hefeweizen and belgian strains, while others have a very clean fermentation profile, like cal ale, with minimal by-products. some are more flocculent than others, etc... hopefully this answers your question. if not, let us know.