From WoWWiki

The Maelstrom is the gigantic rift that was created when the Well of Eternity collapsed at the Sundering of the World.

From the Warcraft III: Reign of Chaos manual:

As the aftershocks from the Well of Eternity's implosion rattled the bones of the world, the seas rushed in to fill the gaping wound left in the earth. Nearly eighty percent of Kalimdor's landmass had been blasted apart, leaving only a handful of separate continents surrounding the new, raging sea. At the center of the new sea, where the Well of Eternity once stood, was a tumultuous storm of tidal fury and chaotic energies. This terrible scar, known as the Maelstrom, would never cease its furious spinning. It would remain a constant reminder of the terrible catastrophe... and the Utopian era that had been lost forever.

As things currently stand, the Maelstrom is a large storm only viewable in World of Warcraft on the world map, where it sits between Kalimdor and the Eastern Kingdoms. Ship and Zeppelin routes go out of their way to avoid the Maelstrom, for fairly obvious reasons. It is implied that the Maelstrom is slowly drawing the Broken Isles towards itself, and will ultimately destroy them for good.

At the center of the Maelstrom is a location known as The Eye. The empires of the Makrura and Naga lie beneath it.

In Warcraft III, when Thrall's fleet crashed, it made an emergency landing on an island near the maelstrom where the trolls were fighting desperately against human colonists and murloc raiders. The island was destroyed (though not by the Maelstrom) and Thrall barely escaped.

To some, the sea around the Maelstrom is known as the "Blood Sea"^{MoM 195,196}

[edit] Zones & Locations

• The Eye
• Nazjatar
• The Boiling Terrace
• The Rift
• Scintal Reef
• The Drowned Reaches
• Gishan Caverns
• Mak'aru
• Pillar Deep