Temperature affects the metabolism of yeast, as well as selects for various characteristics. When making bread, a long, slow rise in the refrigerator allows for more flavor than a fast rise in a warm location.
The taste of hefeweizens can be altered by changing the temperature at which the yeast live and eat - the fermenting temperature. I am lucky, in that there is an unfinished storage room in my basement that stays relatively cool, a perfect place for me to store my fermenting wort. One day I will hack a chest freezer for more accurate temperature control, but since my beers have stayed below 70 degrees Fahrenheit so far, this upgrade may be delayed.
Temperature is important on brewing day as well. Just like with fresh chicken, you want to keep your wort out of the bacterial danger zone, at least until it's got a big gang of yeast cells and is protected from further inoculation. The FDA puts the danger zone from from 40 to 140 F, but advises further caution between 90 and 140. Above 140 F kills bacteria, and if you can keep your wort aseptic as it cools, you have effectively Pasteurized your wort. Unfortunately, these temperatures also kill yeast.
You have to cool your wort before letting your pack of hungry yeast loose on it. The longer you sit in the danger zone, the more bad microbes fall into your cooling wort. These nasties probably won't ruin your beer, because your yeasts will outcompete them, but they will introduce unsavory flavors. New brewers usually cool their wort in an icewater bath in their sink. My brewing buddy happens to be a HVAC Mechanic, and rigged up a wort cooler for our use. This reduces the cooling time from 40+ minutes to about 8 minutes, from boil to 90 F.
I sincerely think that we have been able to brew good beer from the start largely due to the wort chiller and fermentation temperatures.
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