The single most successful group of merchants in the middle east thousands of years ago were the Aramaeans. Their name derives from a region in what is now Syria known as Aram. That is why Aramaic was the lingua franca
of the middle east two thousand years ago. In the so-called New Testament, there are many Aramaic survivals in the Koine Greek versions, both in common nouns and proper nouns, especially place names. The Aramaeans became confessional Jews--which is to say, they practiced the religion of the Jews. In fact, they spread confessional Judaism all over the middle east, and all along their trade routes, and as far east as China. (Many of them later became Nestorian Christians, and they spread their new religion along the same trade routes, once again, as far east as China.) At the time of Mohammed, the most common "non-pagan" confession in Arabia was Judaism, thanks to the Aramaean merchants. That explains to a large extent the heavy influence of Judaism on Islam--whether or not Muslims are willing to recognize it.
The Aramaeans were very successful merchants, and after their conversion, they were seen as Jews. The entire stereotype of Jews as greedy, miserly, and successful merchants because of their sharp practice derives from this Aramaic tradition in confessional Judaism. The stereotype is far older than the irrational hatred of Christians in the middle ages. GW is absolutely correct about the lending of money--other professions in which the Jews excelled were goldsmiths and silversmiths, and pharmacology. Muslims were forbidden to do representational art, and they had no tradition from Arabia for jewelers and pharmacists. Both Christians and Jews were highly valued in Muslim communities (highly valued by the powers that were) as goldsmiths, silversmiths and "druggists," and those professions spread to Europe from the middle east, even long before the crusades. When the city of Fez was founded by a Shi'ite dynasty in Morroco in the 8th century, a special distict was laid out for Jews and Christians so that the rulers could take advantage of their skills, and protect them from common bigotry.
The same thing happended in European cities--in Venice, the Jews were restricted to an island called Ghetto, which was originally a reasonably roomy and not unpleasant place to live. As Jews were persecuted elsewhere in Europe, one place they could flee to was the island of Ghetto, which evenually became overcrowded with an impoverished population, and so the name in that sense stuck. (It apparently derives from an Italian word meaning slag or waste, and that was what the island was originally used for, the storage of slag from metalworking.)
So, as Jews were restricted to very few professions, and many of those were associated with their money lending activities, and that was combined with the older stereotype of sharp practice as merchants which survived from the Aramaeans in the Hellenistic world--the stereotype of the greedy, miserly Jew became endemic in European cultures. Since Jews were most commonly the money-lenders in any give place (but not the only ones--see, for example the Fuggers in Germany or the Lombard--Italian--bankers), if local petty rulers were embarrassed by their debts, they could simply encourage riots against the Jews, and their creditors disappeared. It became such common practice that if a ruler owed money to the German or Italian bankers, he could just borrow money from the Jews, pay off the others, and foment a pogrom against the Jews. The Jews had no friends in high places.
This was particularly true in England; although today people wouldn't think it because of the image of Jewish pogroms in Poland and Russia, the worst excesses in the middle ages were in England. You likely won't know this if you were educated in England. In 1290, Edward the First expelled the Jews from England. Edward had never liked relying on the feudal levy (which had always failed his father, Henry III), and he commonly hired his troops, and when his wars did not return as much money as he spent, he went to the Jews. In the rebellion against his father in the mid-13th century lead by de Montfort, the Earl of Leister and King Henry's brother-in-law, Monfort has convened a Parliament, the first in English history, which was essentially just a "House of Lords" to attempt to gain support for his campaign against the King. Henry died in 1272, and after the expulsion of the Jews, and with his planned campaign against Scotland languishing, Edward convened the first regular Parliament in the 1290s (1295, i believe). But Edward was canny--he also had a House of Commons, which allowed the commonalty--mostly prosperous merchants or knights from the counties--to hobnob with the high and mighty, and to exercise the "privilege" of voting taxes or loans for the King. Edward could also isolate the "overmigthy" lords in a house of their own, were they were happy to approve of taxation on the commonalty, passed by the House of Commons. Edward did not need the Jews.
Jews slipped into England from time to time, and their money was welcome, but it was a dangerous proposition, because it could be all their lives were worth. More than 350 years later, Oliver Cromwell officially allowed the Jews back into England because he wanted to attract capital investment to rebuild England after three ruinous civil wars in a dozen years (1640-1651). After Cromwell died, Charles Stuart was restored as King Charles II in 1660. Not long after, the Jews came to him complaining of extortion and threats by thugs in the east end of London. Charles knew it wouldn't be a good idea to bring that up formally with the Parliament, so he sent some of the boys around to "talk" to those thugs, and the Jews were finally safe (more or less) again in England.
There is a very strong tradition of anti-semitism in Europe and especially in the English-speaking world. Every stereotype, however, can be traced to causes which make a mockery of the ignorance of the Jew haters. More than anything else, it became a pervasive habit in the upper levels of European society to blame the Jews for any of the woes of their communities, and to turn the fury of the ignorant and superstitious on the Jews, and away from themselves.
EDIT: The Jews and the slave trade is purest bullshit--it has been promulgated and spread largely by antisemites among the black community in the United States. Muslims set up the west African slave trade which was later exploited by the Portuguese many centuries after it began, and then very successfully exploited by their commercial rivals, the Dutch. Later, Americans from New England took over--their descendants would still later use the leisure provided by the wealth derived from the slave trade to become abolitionists. Irony is a killer.