In the Germanic spiritual tradtion, Yggdrasil is the cosmic World Tree or "Tree of Life". It is a gigantic tree that rises out of the Well of Wyrd ("Destiny") and gives the universe its basic infrastructure which binds together all of the Nine worlds of the universe.
These Nine Worlds rest within its roots or branches and due to this the World Tree often serves as a conduit for travel between the worlds. Yggdrasil is often spoken of as an ash, though it was thought to have needles like a yew and also bore fruit. More likely than not the tree cannot be compared to any mortal species of tree, but may, indeed be a combination of them all.
The name Yggdrasil literally means the 'Steed of Yggr' i.e. the horse of Odin, since Yggr ( "Terrible One") is one of Odin's many names. This name refers to the nine nights Odin is said to have spent hanging from the World Tree as a self-sacrifice in order to find the Runes, as described in the Havamal (although the tree is not explicitly identified as Yggdrasil):
"I hung on that windy tree for nine nights wounded by my own spear.
I hung to that tree, and no one knows where it is rooted.
None gave me food. None gave me drink. Into the abyss I stared
until I spied the runes. I seized them up, and howling, I fell."
Three roots supported the trunk, with one passing through Asgard, one through Jotunheim and one through Helheim. Beneath the Asgard root lay the sacred Well of Urd ("Fate"), and there dwelt the three Nornir, over whom even the gods had no power, and who, every day, watered the tree from the primeval fountain so that its boughs remained green. Beneath the Jotunheim root lay Mímisbrunnr, the spring or well of Mimir ("Memory"); and beneath the Helheim root the well Hvergelmir ("the Roaring Cauldron").
Ásgard, Álfheim and Vanaheim rested on the branches of Yggdrasil. The trunk was the world-axis piercing through the center of Midgard, around which Jotunheim was situated, and below which lay Nidavellir or Svartálfheim. The three roots stretched down to Helheim, Niflheim and Muspelheim, although only the first world hosted a spring for Yggdrasil.
On the top of the World tree perched a giant eagle, (with a hawk upon its forehead named Vedrfolnir ("wind breather"), who blew the winds over the worlds with his mighty wings. The Niflheim root of Yggdrasil are gnawed at by the dragon Nihogg ("Vicious Blow"). The messenger of the World Tree (and thus between the worlds) is the squirrel Ratatosk who scurried up and down the tree between Nidhogg and the eagle, forwarding insults between them. There were also four stags feeding on the bark of Yggdrasil: Duneyrr, Durathror, Dvalin, and Dainn.
Yggdrasil is also central in the myth of Ragnarok, the end of the world. The only two humans to survive Ragnarok (there are some survivors among the gods), Lif and Lifthrasir, are able to escape by sheltering in the branches of Yggdrasil, where they feed on the dew and are protected by the tree
"The bellowing fire will not scorch them;
it will not even touch them,
and their food will be the morning dew.
Through the branches they will see a new sun burn
as the world ends and starts again."