The Well of Eternity is the title of a book in the War of the Ancients Trilogy.
Ten thousand years before the orcs and humans clashed in their First War, the world of Azeroth cradled only one massive continent, surrounded by the sea. That landmass, known as Kalimdor, was home to a number of disparate races and creatures, all vying for survival amongst the savage elements of the waking world. At the dark continent's center was a mysterious lake of incandescent energies. The lake, which would later be called the Well of Eternity, was the true heart of the world's magic and natural power. Drawing its energies from the infinite Great Dark beyond the world, the Well acted as a mystical fount, sending its potent energies out across the world to nourish life in all its wondrous forms.
From this magical ether the Silithid were born.
(The WarCraft III manual implies that the Titans carefully placed the Well, but the War of the Ancient Trilogy implies that the Old Gods have some relationship to it [Don't worry, all of the World Tree's power came from the Aspects], so we can't be sure.)
Later, Sargeras convinced Queen Azshara and the Highborne to create a portal in the well and leading to the first invasion of the Burning Legion.
Following the War of the Ancients, the Well of Eternity was destroyed, and most of Kalimdor with it, leaving only the Maelstrom. However, Illidan had filled several magical vials with water from the Well of Eternity, and he poured some of the magical water into the lake at the top of Mount Hyjal, creating a new Well. For this insane act, his brother Malfurion and the demigod Cenarius imprisoned him deep beneath Mount Hyjal.
The new Well was completly absorbed by the growth of Nordrassil, its malevolent magicks lost with the enchantments of the Aspects.
However, Illidan had not emptied all of his vials in creating the second Well of Eternity (which is why the Aspects were able to enchant the World Tree with no interference). Somehow, the Highborne under the leadership of Dath'Remar had acquired one of the remaining vials and, after founding their new homeland of Quel'Thalas, used it to create the Sunwell.
Night Elf Origins
It is rumored, but has not been confirmed by an official 
[Blizzard] source, that the Night Elf race was created when a tribe of trolls first came across the Well of Eternity during their travels. They tapped into the Wells magical energies and, over time, were warped into the first Night Elves.