The Cardassian Holocaust was the common name for the near-extermination of the population of Cardassia Prime by the Klingon Empire. Along with the Maquis Holocaust, it was also referred to as one of the "Holocausts of 2372". (Star Trek: Pendragon)
During the early 2370s, the Changeling Founders of the Dominion sought to sow chaos and fear among the Alpha Quadrant powers in order to weaken them as a prelude to invasion. In the Klingon Empire, a Changeling infiltrator posing General Martok convinced Chancellor Gowron that the Cardassian government had been replaced by the Dominion. Under the false Martok's urging, Gowron committed the Empire to war against Cardassia and withdrew from the Khitomer Accords. (DS9: "Way of the Warrior", "Apocalypse Rising")
The Federation initially stood against the Klingon aggression while still attempting diplomatic resolution to the crisis. Once the Federation Civil War broke out, the UFP's attention was diverted internally. As a result, the Klingon assault on Cardassia remained unchecked, and escalated over the course of 2372. By the middle of the year, the Klingons had pushed the Cardassian military back to their home systems. Once the Klingons reached Cardassia Prime, Chancellor Gowron gave an order that surprised the galaxy: instead of putting troops on the ground, he ordered the use of mass drivers to bomb Cardassia from orbit.
During the bombardment, every major city was leveled, and over five billion Cardassians were killed. Cardassia Prime's ecosystem was irrevocably damaged, as the multiple impacts caused catastrophic upheavals in the planet's crust, cracking the mantle and igniting a chain reaction of volcanic eruptions. Within a period of 72 hours, the already ecologically-challenged planet became nearly incapable of supporting life.
Millions of Cardassian survivors tried to flee the planet, many of those were shot down by the Klingon forces, while those that made it through escaped to other worlds in the Cardassian Union and into the Demilitarized Zone. The Klingons pursued the survivors relentlessly, exterminating entire planetary populations and dividing Cardassian territory amongst themselves. By 2374, the Cardassian race had been nearly driven to the point of extinction. (PDN: "Exile", "The Distant Fire", "L'Morte d'Maquis", et. al)