Hebrew is a Semetic language of the Afro Asiatic family whereas Yiddish ( literally Jewish )although written with the Hebrew alphabet is a Germanic language and originated from the Ashkenazi .
Contrary to what people might think, Yiddish and Hebrew are very different languages. The reason why the two are often linked in people's minds is that Yiddish speakers have usually learned how to read Hebrew in childhood, since the Bible and Jewish .prayers are written in classical Hebrew. But this form of Hebrew is very different from the modern Hebrew spoken and written in Israel, which few Yiddish speak�ers speak or understand. The fact is that linguistically Yiddish and Hebrew are as different from one another as Japanese is from Chinese. Just as Japanese borrowed from Chinese its system of writing, along with many Chinese words, so Yiddish borrowed its system of writing from Hebrew, as well as many Hebrew words. And just as Japanese and Chinese are two totally different and unrelated languages, so too are Yiddish and Hebrew.
Yiddish is considered a 'fusion language' and is spoken chiefly by Jews of East European origin or descent. Hebrew is spoken chiefly by Israeli Jews or Jews of Middle Eastern origin or descent.
You won't get better answers than have been given, one rider though, since the creation of Israel, Yiddish, though still spoken as Octavius says, by Eastern European Jews, is dying out, I don't think it will ever die out completely, but now, its mainly the older generations that still speak it.