Junkers were the landed aristocracy in Eastern Germany who wielded great political power throughout the 19th century and well into the 20th through the end of World War II. The Junkers were most prevalent in Prussia, so as went Prussia's fortunes, so too went theirs. As the saying went in the 19th century, "Prussia rules Germany, and the Junkers rule Prussia."
The word "Junker" is a contraction of the German words junger herr, meaning "young lord." The word "Junker" was never an official title with a precise definition, but rather was a colloquial term loosely applied to a certain class of lesser gentry. The term is quite ancient, and was in widespread use by the 16th century, when Martin Luther took the pseudonym "Junker Jörg" while in hiding at Wartburg Castle in 1521. The Dutch version of the word - Jonkheer - is famous as the origin of the New York suburb of Yonkers (which once belonged to a Dutch Jonkheer).
As members of the aristocracy, Junkers typically had particles such as "von" and "zu" prepended to their surnames, and most, if not all, were descended from the Uradel - the ancient German nobility of the early Middle Ages. The fortunes of the Junkers rose and fell over the centuries. Many Junkers lost their lands during the tumultuous years of the 17th and 18th centuries, and were forced to make their living as mercenaries or by entering trade, only to regain their lands again through purchase with the wealth gained from their military and commercial activities.
By the mid-19th century, the Junkers had become firmly entrenched in Prussia, serving as the bulwark of the Hohenzollern Dynasty and completely dominating the Prussian parliament, military, and civil service bureaucracy by effectively marshalling their wealth, connections, and superior access to education to keep commoners out of the upper levels of government. Many of the most famous military and political leaders in German history came from the Junker class, including Otto von Bismarck and Paul von Hindenburg.
Because their power sprung from heredetary priviledges and land rights, the Junkers were consistently a force of conservatism and reactionism against liberal and democratic elements in German society. Although the Junkers were strongly associated with the Prussian monarchy, they survived the dissolution of the monarchy after World War I, such that even under the democratic Weimar Republic most of the higher leaders came from Junker families.
The Junkers began to decline under Adolf Hitler, although not precipitously so. Although Hitler was of course not from a Junker family and was known to express his disdain for Junkers from time to time, many Junkers supported him because they hated democracy more, and so while Hitler did not specifically promote the interests of the Junkers he never took any special action to dispossess them of their extensive landholdings and many Junkers continued to hold influential positions, especially in the tradition-bound German military.
The final destruction of Junker power came in the aftermath of World War II. Since the vast majority of Junker landholdings were in what became East Germany, they were hit hard by Communist land-redistribution policies, as approximately 90 percent of Junker lands were reorganized into communal farms or confiscated by the state for other purposes. Since the reunification, some of the old Junker families have tried to reclaim their old estates in the German courts, but these attempts have not succeeded.