The Great Sundering, also known as simply the "Sundering" or the "Cataclysm" (not to be confused with the cataclysm caused by Deathwing), was a world event which reshaped Azeroth approximately 10,000 years ago at the end of the War of the Ancients. This was caused when Queen Azshara created a portal for Sargeras in the Well of Eternity. The spell created an unstable vortex within the Well's turbulent depths. When those channeling the portal were attacked at the Eternal Palace, it threw the carefully crafted spellwork into chaos.
The unstable vortex within the Well's depths exploded and ignited a catastrophic chain of events that would sunder the world forever. The massive explosion rocked the temple to its foundations and sent massive quakes ripping through the tortured earth, as the surging Well of Eternity buckled in upon itself and collapsed.
The implosion of the Well created a vortex that began to consume the central landmass of Kalimdor, and the seas rushed in to fill the gaping wound left in the earth. The continent had been blasted apart into smaller continents.
Prior to the Sundering, there was only one known continent on Azeroth, referred to as Kalimdor. In the catastrophic explosion, eighty percent of the land mass was destroyed leaving behind the major continents and scattered islands that are known today: Northrend, Kezan, the Eastern Kingdoms, Pandaria, and the remnant still referred to as Kalimdor.
The site of the Well of Eternity became a swirling vortex of power known as the Maelstrom, a storm of chaotic energies, which continues to rage to this day.
The concept of a sundering, or a world being reshaped, is a common theme found in fantasy. Tolkien described a reshaping of Arda, and lands lost to the seas in his writings, and the Dragonlance Dungeons and Dragons gameworld underwent a Cataclysm, where seas receded to the point that a coastal port town became landlocked. In Warhammer, the Gates of Old Ones once shattered and Chaos swept the world, damaging the land, primary on north and south poles. In David Eddings' Belgariad, the dark God Kal Torak attempted to use a magic orb that had been used to shape the world in anger and split the world in two and mutilated himself, an event similar to the Sundering.
Then, Sageras said to his loyal demons: "FUCK MY GREAT ASS!"