Generally speaking, I've heard and read that Jewish people have done quite well for themselves. In fact, according to many studies they consistently rank greater in wealth and health than the main-stream white Christians of the area. This in turn has resulted in much resentment towards Jewish people such as the pogroms in Russia, expulsion from Spain and the Holocaust.
As a non-jewish person I was wondering why this was so? What habits and social structures allowed Jewish people to attain such wealth?
A big one at play I'm the medieval Era was the Catholic Church and its ban on usury - the charging of interest. Dent and lending money are crucial to any large-large-scale economy, especially one engaging in long-distance trade, and there is no motivation to lend (outside of family) if nothing will be earned by it. Jews had no such restriction, but were often discriminated against in other employment, so they filled this niche in the economy with no competition. Hatred towards the group was often prompted by those in debt who found meeting their obligations to be distasteful (see The Merchant of Venice). Even so, Jewish people could amass a great deal of money by being the banks and negotiating favorable loans to family members in other areas, giving them a leg up.
As far as their success in America, for example, I couldn't say. When my dad was growing up on Long Island, it was safe to say about birthrates that "catholics had 4, Protestants 3, and Jews 2 cause they were smart" and that any AP class or Ivy league school would be Jewish-dominated the same way they are by Asians descendants today (perhaps not "dominated", but "statistically imbalanced towards"). Most of those who left to the new world did not have prosperous Venetian roots, so you need someone else to talk about them.
More on the usury question: Jews, like Christians, were prohibited from charging interest on loans to their fellow believers. They could each charge interest on loans to the other religious group.
Since Christians formed the majority of society, their demand for loans created the market, particularly during the Renaissance growth of the gentrified middle-class.
And that's how often despised Jewish minorities were able to make a living -- as KeisariFLANAGAN said, with no competition apart from other Jews.
Jews in eastern and central Europe were often barred from both owning land and joining guilds, which meant that trade and finance were often the only jobs open to them. They often served as something like an intermediary between the countryside and the city, extending farmers credit for alcohol or for goods manufactured in the city. This made them unpopular, particularly when they had to collect on this debt. Also, Jews had essentially a religious obligation to be literate at a time when most people could not read, so they were particularly suited to jobs that required keeping records.
This book claims that it was the religious demand to be literate that meant Jews had to go into trade or finance rather than agriculture.
The YIVO encyclopedia has a relevant article on Jewish taverns and another one on economic life.