(Also The World Tree, Ask Yggdrasil, Hoddmimir's Wood, Laerad or Odin's Horse)
"The Terrible One's Horse"
"The Eternal Tree of Life"
"The Tree of Knowledge"
"The Horse of Yggr*"
In Norse mythology, Yggdrasil, the world tree, is a giant ash that links and shelters the nine worlds: Asgard, Ljossalfheim, Mitgard, Svartalfheim, Hel,
Jötunheim, Muspelheim, Vanaheim and Niflheim. The tree itself, created many years before the earth was formed, was the giver of all life, but also symbolically important. As long as Yggdrasil stood, healthy and green, the gods would be safe from Ragnarok.
The tree is supported by three roots. One root is planted in Asgard, one in Jötunheim (where Ginungagap was once located), and one in Niflheim. Under the root that extends over Niflheim is the spring Hvergelmir. The rivers Svol, Gunnthra, Fjorm, Fimbulthul, Slidr, Hrid, Sylg, Ylg, Vid, Leiptr, and Gjoll all originate from that spring.
Under the root which reaches into Jötunheim is Mimir’s well (Mímisbrunnr), which has all wisdom and intelligence contained within it. Mimir is its guardian, and Odin paid the price of an eye for the privilege of a drink.
The root that extends into Asgard sits atop Weird’s well (Urdarbrunnr or Nornesalen, the holiest of wells). Each day, the gods ride over Bifrost, the rainbow bridge, to a beautiful hall by the well where they hold court. It is there that Weird, Verdandi and Skuld (the three Norns, the weavers of the web of fate) take water from the well and pour it over Yggdrasil to prevent it from death and decay. The water in Weird’s well is so holy as to turn everything that touches it white (the surrounding mud and the feeding swans are the color of eggshells).
There are animal inhabitants of Yggdrasil. Nidhogg, a serpent, lives under the root into Niflheim. He gnaws at the tree and, with the aid of innumerable snakes, torments the dead. Four stags (Eiktyrne, representing the four winds) also torment the World Tree: Dain, Dvalin, Duneyr and Durathror all run through the branches and feed on the leaves. Messages are delivered between worlds by the squirrel Ratatosk ("swift teeth"), and a eagle or golden cock, Vidofnir ("tree snake") sits perched on the topmost bough.
On the day of Ragnarok, Surtr, the fire-giant, will set fire to the tree, and it will be destroyed. According to some versions of the great battle, Lif and Lifthrasir (the two humans that survive Ragnarok) stay alive by hiding in the branches of Yggdrasil.
Sources:
http://www.angelfire.com/on/Wodensharrow/yggtexts.html
http://www.pantheon.org/articles/y/yggdrasil.html
* Yggr is one of the many names of Odin