Pumpernickel is a typically heavy, slightly sweet rye bread traditionally made with sourdough starter and coarsely ground rye. It is sometimes made with a combination of flour made from rye as well as whole rye grains ("rye berries").

At one time it was traditional peasant fare, but largely during the 20th century various forms became popular through delicatessens and supermarkets. Present-day European and North American pumpernickel differ in several characteristics, including the use of additional leaveners. The North American version may have coloring and flavoring agents, added wheat flour, a higher baking temperature, and a dramatically shortened baking time.

Rye is a grass grown extensively as a grain, a cover crop and a forage crop. It is a member of the wheat tribe ('Triticeae') and is closely related to both wheat ('Triticum') and barley (genus 'Hordeum'). Rye grain is used for flour, bread, beer, crispbread, some whiskeys, some vodkas, and animal fodder. It can also be eaten whole, either as boiled rye berries or by being rolled, similar to rolled oats.

More Info: en.wikipedia.org