What exactly happened during the Progrom?

Everyone knows about the Holocaust and the Spanish Inquisition but no one speaks about the Progrom much. I have jewish ancestry and from what my dad has told me the Progrom was kind of like the Soviet genocide of jews. My dad told me that in the Ukraine and Russia they used to tie rabbis by their legs and arms to two horses, they would get the horses to run in opposite directions to basically rip the rabbi's body in two. My dad told me there were many disgusting atrocities committed by the soviets that were much like that. What I want to know though is what exactly started the Progrom?