title_body: What the heck is a lambic?
upvoted_answer: For how lambics are made and what distinguishes them from other beers, see this Wikipedia article.  To summarize, lambics are exposed to wild yeast (rather than the usual cultivated yeasts) and tend to have a dry, tart taste.  These are not heavy-bodied beers, nor are they bitter.  (Hops are used for their preservative effect, but old, dry hops are used to minimize the flavor impact.)
  
  Frequently, lambics (like the one you had) have fruit added later in fermentation.  Common fruits for this include raspberry, peach, and cherry, and I've also encountered apple.  The sweetness you're tasting probably comes from the fruit (and some of these varieties taste sweeter to me than others).  If you'd like to try a lambic that doesn't have the fruit, which is likely to be less sweet, look for "gueuze".