The nature and significance of Yggdrasil is recorded in the Poetic Edda, compiled in the 13th century from earlier sources, and in the Prose Edda written in the 13th century by Snorri Sturluson. Both record Yggdrasil to be a great and holy ash tree--the entire world, located at the center of the universe.
The Tree is living, with branches extending above the heavens and a trunk firmly latched to the ground by three great roots. From its center span the nine realms of existence, including Asgard, the home of the gods and the hall of Valhalla, and Midgard, the land of the human mortals. An eagle sits in the top of its branches in constant conflict with an evil serpent, Nidhogg, who tries to unbalance the tree by chewing through one of its roots.
Trees are prominent in Norse lore elsewhere too. Both men and women were created from a tree. Man, Ask, was named from a derivation of the word askr, which is literally translated as ash tree. Woman, Embla, was said to have been made from an elm tree.