Low German is the language of northern Germany. But it can also be heard in the north-eastern Netherlands, in the south of Denmark, and even on other continents! It is often scolded as a mere dialect, but Low German and Standard German parted ways around 1500 years ago. Though they never went far apart.
In the late Middle Ages, Low German was the language of the Hanseatic League and could be heard as such from Iceland and Great Britain across Scandinavia and the Baltic States to Russia. The language was in use all around the North Sea and the Baltic Sea, and traces of Low German can still be found today in all the languages around the two seas.
Today, on the other hand, the Low German language has a difficult time. In its prime, Low German probably had over 25 million speakers; today only a fraction of northern Germans can actually speak Low German.
The language differs from Standard German first of all through the High German consonant shift. This sound shift was made by High German, while Low German stayed the same.
The sound shift created the words “Apfel” and “Schiff” from “Appel/apple” and “Schipp/ship” (p → pf / ff), from “ik/I” the word “ich” (k → ch), from “Tiet/tide”, “Katt/cat” and “dat/that” the words “Zeit”, “Katze” and “dass” (t → z / tz / ss) and from “Dag/day” the word “Tag” (d → t). If you compare these words with their English and Dutch equivalents, it is easy to see that Standard German has evolved away from the other three languages.