The word 'Pogrom' is derived from the Russian 'to wreak havoc', to riot or cause devastation1. It is most often applied a series of mob attacks on Jewish people and their property in the late 19th and early 20th century.
The first modern Pogram is considered to be the Chmielnicki Pogrom2 of 1648-52 in which over 100,000 Jews are thought to have perished. The Ukranian officer Chmielnicki led the Orthodox Cossacks and peasants against the Jews, driving them west and pursuing them into Poland.
In 1881 the Czar Alexander II was assasinated by radicals. However, Jews were blamed. That year there were 200 individual Pogroms. The anti-Semitic government turned a blind eye but probably did not directly organise the violence. Jewish businesses were ruined and many were left homelesss, injured or dead.
In 1903 another violent outpouring of hate over three days resulted in 49 deaths and 700 homes looted in Kishinev, Moldova as a garrison of troops stood idle. This Pogrom started after the murder of a Christian child was blamed on Jews (it turned out the child had been murdered by a relative) and the suicide of a Christian woman in a Jewish hospital (unrelated to Jews). It is frightening to note how easily mass hysteria can take hold of a population and inflame them to commit acts of irrational violence.
During World War I hundreds of thousands of Jews were deported to central Russia. During the Russian Civil War, the White Russians continued to encourage Pogroms and spread hateful propaganda.
Pogroms resumed in force during the thirties in Hungary, Mexico, Poland, Rumania and Germany. During the Holocaust, the pogroms were meticulously planned beginnning with Kristallnacht (1938). The government of Nazi ally Romania organised their own atrocity in 1941. Even in post war Poland (1946)there was a Pogram in Kielce.
The term 'Pogrom' is also applied to acts of mob violence against other minorites. Hence the Turkish Pogrom against the Greeks of Constantinople/Istanbul in 1955.
2Kudos to arieh for pointing this out. Info also from www.ou.org/about/judaism