Yiddishthe language of East European or Ashkenazic Jews. Yiddish is to be distinguished from Hebrew, which is the language of Jewish prayer and the official language of Israel.
Yiddish is descended from the form of German heard by Jewish settlers who came from northern France to Germany a thousand years ago. Almost all East European Jews before WWII understood Yiddish and many would have spoken it at home. Some East European Jews attended schools taught in Yiddish. It was estimated that in the 1920's about two-thirds of the Jews in the world could understand Yiddish. Although Yiddish used to be a lingua franca or common language for Jews, this is not true today.
Yiddish possesses an incomparable vocabulary to express shades of feeling and a rich storehouse of characterization names, praises, expletives and curses. A Yiddish literature developed that matured into a diverse and sophisticated body of work. Perhaps its most famous exponent was Sholem Aleichem (the pen name of Sholom Rabinowitz 1859-1916). The musical "Fiddler on the Roof" is based on his stories. YIVO, the Yiddish Scientific Institute, was created in Vilna and continues in New York City to further Yiddish research and culture.
The Holocaust had a devastating effect on the Yiddish language because it destroyed most of its speakers. Isaac Bashevis Singer(1904-1991) won the 1978 Nobel Prize for literature for his Yiddish novels and short stories. In 1980, the National Yiddish Book Center opened in Amherst, Massachusetts.
Source: Rosten, The Joys of Yiddish.