The Diaspora - dispersal of Jewish peoples - was significant during the Roman era. They left because the Romans treated them harshly. The Romans were moderate in their treatment of cooperative people. Rebellious kingdoms received violent retribution. The Jews were bitter toward the Romans and non-cooperative.
Jewish peoples emigrated to every corner of the globe, but most of them went to Europe. Nations famous for their influential Jewish populations were Italy, Spain, France, Germany and Russia.
Forced to live in the back of town, the Jews kept themselves generally separate from the Christians. No Jew would consider a marriage with a Christian. They lived in the ghetto, an Italian word for the Jewish Quarter.
Christians - including the Popes - encouraged mistrust and hatred of Jews. They blamed Jews for the death of Jesus (deicide) and opposed them for not accepting Jesus as Messiah.
From Wikipedia on anti-Semitism in the Middle Ages in Europe:
Among socio-economic factors were restrictions by the authorities, local rulers and frequently church officials who closed many professions to the Jews, pushing them into marginal occupations considered socially inferior, such as local tax and rent collecting or moneylending, seen in these times as a "necessary evil" due to the increasing population and urbanization during the High Middle Ages. Catholic doctrine of the time held that moneylending for interest was a sin, and as such Jews tended to dominate this business. The Torah and later sections of the Hebrew Bible criticise Usury but the Bible is slightly ambiguous. Jews were forced into money lending as there were few other occupations open to them. This provided support for claims that Jews are insolent, greedy, engaged in usury, and in itself contributed to a negative image. Natural tensions between creditors (typically Jews) and debtors (typically Christians) were added to social, political, religious and economic strains. Peasants who were forced to pay their taxes to Jews could personify them as the people taking their earnings while remaining loyal to the lords on whose behalf the Jews worked.
Today, usury means loaning money at exhorbitant interest.
Also:
As the Black Death epidemics devastated Europe in the mid-14th century, annihilating more than a half of the population, Jews were taken as scapegoats. Rumors spread that they caused the disease by deliberately poisoning wells. Hundreds of Jewish communities were destroyed by violence, in particular in the Iberic peninsula and in the Germanic Empire.
Torah, the Jews’ scripture - usually read from a traditional scroll.
Pogrom - murderous massacre of a Jewish community.