As they say in Wikipedia-land: this page is for disambiguation.
Yiddish is a language based mainly on a combination of German and Hebrew. Yiddish is written with the letters of the Hebrew alphabet. Dialects of Yiddish include words from other languages such as Polish, Russian, Latvian, etc.
Yiddish is considered by many to be a dying language, due to the Holocaust, when most of the world’s Yiddish speakers were murdered.
Yiddish is still spoken by enclaves of Jews in parts of New York, Jerusalem, and other cities around the world that have sizeable communities of ultra-orthodox Jews.
Yiddish is making a (small) comeback among some younger Jews who want to preserve and revive Yiddish. See also the National Yiddish Book Center at http://www.yiddishbookcenter.org/