Yggdrasil (イグドラシル) hosts the entire Digital World.
In Digimon Elements Yggdrasil, the World Tree, holds the entire Digital World together. It is it's core and practically fills up the entire world.
The Digital World is in the communications network of the human world. It is populated by Digimon.
In Digimon Elements the Digital World consists of nine layers wich go from the Valaheimr in Yggdrasils branches to Niflheimr in it's roots. Digimon inhabit all of these layers but most of them live at the bottom of the trunk in the Midgard layer.