For most of their history, Jews have been multilingual. Hebrew is the language of the Bible, the principal language of Jewish liturgy, and the language spoken in modern Israel--but it has been the primary language of only a small percentage of Jews who have ever lived.
The geographical diversity of the Jewish people accounts for its multilingualism. Jews have adopted the various languages of their homelands and also spoken numerous Jewish hybrid languages.
By the beginning of the Common Era, Aramaic had replaced Hebrew as the spoken language of Palestinian Jews. The causes of Hebrew's decline are not wholly understood, but it was certainly hastened by the Babylonian exile in 587 B.C.E. and the continued foreign rule of Palestine during the Second Temple period. Aramaic, like Hebrew, is a Semitic language, and there are many similarities between the two.
Because of Aramaic's prominence during the rabbinic era, it is arguably the second most important Jewish language--though it was spoken by non-Jews as well. The Talmud is written in Aramaic, as is the Zohar, the great medieval mystical text. One of the most well known Jewish prayers, the kaddish, also is written in Aramaic. During the talmudic era, Hebrew illiteracy was so high that the Shabbat Torah reading was recited along with a verse-by-verse translation into Aramaic.
Jewish hybrid languages have existed for more than two millennia. Linguists have long puzzled with little resolution over whether these tongues should be considered dialects, unique languages, or Creole languages (languages that began as pidgins--simplified forms of speech, often mixtures of two languages--and are later adopted as primary languages).