What happens when you use different types of yeast? Does it change the flavor? I'm not talking about mixing different types of yeast in the same recipe.
Different yeast does do different things when fermented in the same beer.
Flocculation - Some yeast has high flocculation, which means that it collects or gathers together (I think of a flock of birds gathering). High flocculation is often desired because it leaves a clearer beer. However some beer styles want a low flocculation yeast (IE Hefeweizen).
Attenuation - is the amount of available sugars that the yeast will consume.
Temperature Range - can play a part in which yeast you use. General rule of thumb is that lager yeasts ferment at lower temperatures and ale yeasts at higher.
Alcohol Tolerance - some yeasts don't stand up to the higher gravity beers such as Belgium tripples or Barleywines.
Flavour - The type of yeast does play a part in the flavour of the beer. When the yeast doesn't flocculate well and is suspended in the beer you will notice a yeasty/bready flavour. But even when it does get removed you will still notice different flavours from different yeasts. Certain yeasts will ferment cleaner with no fruity esters, while others will ferment leaving a lot of esters. Some yeasts will not be prone to diacetyl whilst others are.
For more information:
Yeast is one of the most important components of a beer recipe. If you think about it, you do not make beer, you simply provide a friendly environment for yeast to make the beer for you.
Different strains of yeast have evolved over the history of brewing, adapting to their breweries and being selected for different characteristics by brewers. The strain of yeast you choose can profoundly affect the balance of the beer just by virtue of the mix of alcohols and esters that they produce.
Every strain is different, but speaking in broad generalizations and bearing in mind that the flavors described may be subtle:
Belgian yeasts emphasize "spicy" esters. (E.g. White Labs 550 Belgian Ale)
English yeasts emphasize "fruity" esters. (E.g. Wyeast 1275 Thames Valley)
German yeasts tend to emphasize the malt. (E.g. White Labs 830 German Lager)
American yeasts tend to have a "clean" character that emphasizes hop flavor. (E.g Wyeast 1056 American Ale)