Yggdrasil is located in the center on the Norse cosmos, the nine realms circle around it, and are kept together by its branches and roots. That being said, the well being of the cosmos depends on the well being of Yggdrasil.
According to legend it is very windy and freezing cold at the top branches of Yggdrasil, as they are unimaginably high, and the roots go so deep into the underworld no one knows where they end. Its branches and roots are so confusing that no one but the shamans at the end of their life can see where they lead (and predict the future with that knowledge).
There are 3 main large roots: one that connects to Midgard (the world of humans), one that connects to Hel (the underworld) and one to Jotunheim (realm of Giants/Devourers)
Several beings live in Yggdrasil, among them a dragon, snakes, one eagle, four stags and a squirrel. They slowly nibble the tree and wear it off, symbolizing mortality.
There are two versions of the wells of Yggdrasil, one where there is only one, The Well of Urd (Fate) located bellow its trunk; in the other there are 3, the well of Urd is at the top, where the Gods hold Council, another in Jotunheim where Mimir, the wisest Giant pours and observes its knowledge, and the third one called " Hvergelmir " below the root that stretches to Nifleheim (realm of ice).
It is said that when Yggdrasil trembles it will signal the arrival of Ragnarok, the destruction of the cosmos.