World Tree refers to one of the great trees in Azeroth:
Nordrassil, which means "Crown of the Heavens", was the original world tree planted on the summit of Mount Hyjal after the War of the Ancients. It was blessed by the Dragon Aspects to grant immortality to the night elves and to allow their druids to enter the Emerald Dream. It was destroyed after the Battle of Mount Hyjal.
Located on an island off the coast of northern Kalimdor, Teldrassil - which means "Crown of the Earth" - was an unsuccessful attempt by Archdruid Fandral Staghelm and his compatriots to restore the night elves' immortality, lost with the destruction of Nordrassil.
Vordrassil was a failed attempt at a World Tree, located in the Grizzly Hills of southern Northrend. Destroyed by the ancient druids, it is now the site of the furbolg city of Grizzlemaw. The Old God Yogg-Saron sleeps beneath its roots, and appears to have been responsible for the corruption of the furbolg ( [75G] ).
Other Possible Trees
Another possible World Tree shown in a piece of concept art titled Idrassil and is found in The Art of World of Warcraft. However, it is probably just an early version of Teldrassil. The World Tree known as the "Great Tree of Life" appeared in the official Azure Tower Defense map for the Frozen Throne.
The idea of a World Tree is almost certainly a reference to the World Tree of Norse mythology, Yggdrasil. Their names are similar, they both have wells underneath them: the Well of Eternity beneath Nordrassil, and the Well of Urd beneath one of the roots of Yggdrasil. Also, a dragon, Nidhoggr gnawed at the roots of Yggdrasil, which could reference back to either Deathwing or Archimonde. These parallels only apply to Nordrassil, the "true" World Tree, and not Teldrassil, the "false" one. (The idea of a beast gnawing at the roots may have some sort of connection to Yogg-Saron and Vordrassil as well.)