The Jews expelled from Castile and Aragon in 1492 left with the Spanish they had been speaking, and in the cities that received them, above all Salonica, Istanbul, Izmir, Sarajevo and Sofia, that Spanish stopped changing in step with the Spanish of Spain. What resulted is Ibero-Romance in its bones. A Spanish speaker today can follow a good deal of it, but hears forms that Castilian has long since dropped and pronunciations that Castilian moved away from in the sixteenth century.
Around that older core the language accumulated what its speakers lived among. Hebrew and Aramaic supplied the religious and communal vocabulary, as they did for Yiddish. Turkish, Greek and South Slavic supplied the words of the market, the household and the state. Italian entered through Livorno and the Adriatic trade, and French entered late and heavily through the schools of the Alliance Israelite Universelle, which taught a generation to regard their own language as a corrupted dialect.
For most of its history it was written in Hebrew letters, in the semi-cursive type that printers call Rashi script and in the handwriting known as solitreo. Latin and Cyrillic alphabets came in during the twentieth century, and the Latin spellings now used are largely phonetic.
The communities that spoke it were destroyed in the Holocaust, above all at Salonica, and the survivors dispersed. What remains is a heritage language: studied, sung, printed and broadcast, but not acquired by children.