Throughout the centuries, Jews have suffered persecution. The El male rahamim was written for martyrs in the Middle Ages. The post-Second World War version includes the names of several concentration and extermination camps where many Jews were murdered. For the reciting of this prayer traditionally a minimum of ten Jewish men (a minjan) must be present.
See also Kaddish.
You can listen to an excerpt from the prayer on this website: Yizkor.