Yggdrasil = the World Tree, an ash. The following is a quote from Gylfaginning, by Snorri Sturluson:

"The ash is the best an greatest of all trees; its branches spread out over the whole world and reach up over heaven. The tree is held in position by three roots that spread far out; one is among the Æsir, the second among the frost ogres where once was Ginnungagap, and the third extends over Niflheim, and under that root is the well Hvergelmir; but Níðhögg gnaws at that root from below. Under the root that turns in the direction of the frost ogres lies the spring of Mímir, in which is hidden wisdom and understanding; Mímir is the name of the owner of the spring. He is full of wisdom because he drinks [water] from the spring out of the horn Gjöll. All-father came there and asked for a single drink from the spring, but he did not get it until he had given one of his eyes as a pledge."…"The third root of the ash tree is in the sky, and under that root is the very sacred spring called the Spring of Urð. There the gods hold their court of justice. The Æsir ride up to that place every day over the bridge Bifröst, which is also known as the Bridge of the Æsir."

"In the branches sits an eagle, and it is very knowledgeable, and between its eyes sits a hawk called Veðrfölnir. A squirrel called Ratatosk springs up and down the ash tree and conveys words of abuse exchanged between the eagle and Niðhögg. Four harts leap about the branches of the ash and eat the shoots; these are their names: Dáin, Dvalin, Duneyr, Durathrór. And along with Niðhögg in Hvergelmir there are so many serpents that no tongue can count them."

Gylfaginning = "The Deluding of Gylfi" Presumably in an attempt to keep the old religion alive, Snorri Sturluson (1179-1241) wrote this story as the first part of his Prose Edda. Gylfaginning was written in Icelandic about 200 years after the Christianization of Iceland, and it is essentially a guide to Scandinavian mythology. King Gylfi of Sweden disguises himself as an old man named Gangleri and travels to Ásgarð seeking knowledge. There he meets with High One, Just-as-high, and Third, who tell him all there is to know of the gods and their history, from creation to Ragnarök.