Low German (Plattdeutsch or Niederdeutsch), also called Lower Saxon, is a language encompassing the Saxon dialects of the northern part of Germany and a few border provinces of the Netherlands. Although considered a variation of German, in many aspects it is more closely related to Dutch, which is based on closely related Low Frankian dialects.
High German by contrast was mostly based on more Southern dialects that had undergone the second germanic sound shift of the early middle ages. Low German was once much more widespread than today, being used as a lingua franca throughout the Baltic region. It served as a standard language in many regions of Northern Germany until its was replaced in that function by High German during the unification of Germany under Bismarck in 1870. A few regions in the north (Kleve e.g.) saw Dutch replaced by High German instead.