Anti-Semitism: prejudice against the Jews, referred to by some people as the longest hatred. It has been around since Ancient Times, and has appeared in several forms (and thus becomes complex). The forms which comprise Anti-Semitism consist of the following: religious, economic, ideological/political, social, and cultural.
Religious Anti-Semitism began in Ancient times with the Hellenistic culture, and would reach its peak with the Great Revolt against Rome in 70 A.D. (after which the Jews were dispersed throughout the Roman Empire). When Christianity became the dominant religion during the Middle Ages, some Christians became prejudiced towards the Jews, accusing them of being Christ-killers (disregarding the fact that the Crucifixion would bring about salvation for all, not to mention that only a small portion of Jews at that time wanted Jesus to be crucified). Such hatred turned violent at times.
Meanwhile, it was even worse in the Muslim world, in which Jews (and other non-Muslims) became persecuted minorities, having to pay the jizya in return for protection. Such religious hatred faded in the centuries after the Medieval Period (particularly in the West) but has not vanished.
Economic Anti-Semitism also originated during the Middle Ages, when job discrimination led some Jews to finding work as merchants and moneylenders- the latter of which resulted in furthering hatred towards Jews during that period (i.e. Jews were charging interest, in fact being accused of charging high interest out of greed when loaning money). From the nineteenth century through today, Jews became vilified for their alleged control of the global economy, and henceforth, the world. Such accusations were the result of many Jews finding occupations in education, business, banking, and journalism.
Ideological/political Anti-Semitism began during the nineteenth century as the result of some Jews pursuing liberal, even radical ideologies. Karl Marx is a case in point. His ideology of Marxism led to totalitarian regimes, as well as the Cold War. When the Bolsheviks seized power in Russia, many Jews welcomed the revolution because they thought it would bring an end to Anti-Semitism there. As a result, communism was blamed by some people (e.g. Hitler) on the Jews.
Today, the politics of Jews is a sort of paradox. In the West, most Jews have a tendency to support liberal political parties, despite the fact that such political parties are anti-Israel, in addition to gaining most of the Muslim vote (who in turn have a tendency to be Anti-Semites).
Racial Anti-Semitism also originated in the nineteenth century (although some claim it started during the Middle Ages) with nationalism and the myth of the Aryan race. Hitler and the Nazis adopted this myth, and it resulting in the Holocaust. White supremacists elsewhere adopted this concept, as do some Muslims.
Social Anti-Semitism is based on stereotypes. Examples consist of Jews being rude, pushy, or haggling, Jewish mothers and/or wives being overbearing, smothering, or nagging, and occupation (e.g. doctors, lawyers, business and banking, celebrities). Although the personality traits can be found in any group of people, the stereotype dies hard. As for the occupation stereotype, statistics on how many Jews are in such occupations would be a way to refute it.
Finally, there is cultural Anti-Semitism. Throughout the centuries, Jews usually stood out in a crowd (e.g. appearance, customs). That began to change in the nineteenth century with the emergence of Reform Judaism and Conservative Judaism (thus resulting in many Jews quietly assimilating into their communities). However, Orthodox Jews continued to maintain the original customs and appearances (e.g. payot) which led to some people having stereotypical images of Jews.
Of course, all of these Anti-Semitic views are nonsense–in fact, logic and morality can refute such views. Unfortunately, they do not go away easily.
Image: Courtesy of: http://www.polderdash.com/2011/05/hasidic-jews-erase-clinton-from-situation-room-photo/