Pumpernickel is a dense sourdough rye bread made from coarsely ground rye flour, and sometimes filled out with whole rye berries. It is baked very slowly, from 16 to 24 hours, at a very low temperature (250°F / 120°C) in a steam-filled oven. This results in a very dark brown color throughout the loaf, darker than the crust of most breads. It is associated with the Westphalia region of Germany, where it was traditionally eaten by peasants, but it is currently considered a quite respectable deli bread and is used in hoity-toity hors d'oeuvres.
Those of us who live in America are most likely to encounter modified pumpernickels, with recipes that include wheat flour, higher baking temperatures, and added coloring agents and sweeteners.
Pumpernickel is also noteworthy because it is a funny word. But not half as funny as its backstory. Pumpernickel was originally a rather silly German word for a stupid or obnoxious person; it comes from the combination of the name Nickel, literally meaning Old Nick (AKA Satan) and pumpern, an onomatopoetic reference to breaking wind. In other words, a gas demon. Before the bread won this name, it was referred to as krankbrot, literally 'sick-bread'. Both of these names were probably given in reference to its indigestibility when eaten as a central part of one's diet.