The Maelstrom is the gigantic rift that was created when the Well of Eternity collapsed into itself during the Great Sundering.
Warcraft III: Reign of Chaos
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.
At the center of the Maelstrom is a location known as the Eye. The empires of the makrura and the naga lie beneath it.
In Warcraft III, when Thrall's fleet crashed, they made an emergency landing on an island near the Maelstrom where they found a group of trolls fighting desperately against human colonists and murloc invaders. The island was destroyed (though not by the Maelstrom), with Thrall barely escaping.
It is implied that the Maelstrom is slowly drawing the Broken Isles towards itself, and will ultimately destroy them for good.
World of Warcraft
The Maelstrom is a large storm in the center of 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. You must pass through the Maelstrom to get to Deepholm via a quest given from the Earthen Ring in your faction's home city.
In World of Warcraft: Cataclysm, Deathwing re-enters Azeroth from Deepholm via the Maelstrom. The Maelstrom is a sanctuary, where PvP combat is disabled. The zone can be reached through the breadcrumb quests leading to Deepholm from Stormwind and Orgrimmar. Thrall and Aggra are also present here (now affiliated with the Earthen Ring). Thrall is trying to maintain the rift, while Aggra provides transportation down into Deepholm.
Where to start? Well, somebody would say all the way back to Warcraft I (aka Warcraft: Orcs & Humans or the First War), or maybe Warcraft II (aka Tides of Darkness or the Second War up to and including Wrath of the Lich King, Warcraft III is the best place to start. Also, this game then elicits more interesting questions. For instance, how Illidan Stormrage became so hated or where the Scourge started. You will learn all the history and geography of the Warcraft with our help and dedication.