The essential counter-balance to sweetness!
Bitterness is essential for balancing sweetness in beers. Hops are used for their bittering qualities, especially apparent in stouts where you desire little or none of the fruity or resinous character from the hops.
Bitterness can also be found from roasted or smoked and from special malt varieties that bring different characteristics to the beers. Use of wild yeasts is another way to achieve bitterness, bringing desirable earthy, characteristics that brilliantly marry with sharp tangy fruits.