title_body: Why are some beers warm and some cold?
upvoted_answer: I don't think anyone would deliberately or professionally serve warm beer, if by warm you mean something close to body temperature. Generally speaking, cask ale (as opposed to pressurised keg beer) is served, in the UK at least, and according to CAMRA's website, at cellar temperature: 12-14 C (54-57 F), whereas lager and keg beer is usually served much colder.
  
  So much for the purist. My preference is always for cask ale to be served cool, but not cold, so that the flavours can be properly appreciated. Good quality lager should be served colder, and I prefer Czech pilsners at around 6-8 C - about the same temperature as you'd want white wine to be. But on a burning hot day in the African sunshine (where I'm from), there is nothing as refreshing as an ice-cold (and I mean ice-cold) ordinary mass-market lager, straight out of the bottle. You can't distinguish tastes as readily at those temperatures. Refreshment is the key there: you don't really care about taste in those circumstances (if you did you'd be drinking something else).