It is a variety of High German which was originally spoken by Jews living in Germany, where large Jewish communities existed since XII-XIII centuries. Later on, when some of German moved to the east—to Poland, Russia etc.,—they still spoke their version of German language. Yiddish actually means "Jewish", and the language was sometimes called "Jewish German" in the past. Over time, the language borrowed a lot of words, phrases and grammatical constructions from Russian and other Slavic languages and, of course, from Hebrew. I heard that a person with a good command of German can understand most of spoken Yiddish. Reading is a different matter, because Yiddish is written with Hebrew letters.Originally Posted by MoZeS