The Netherworld between Good and Evil is a plane of existence between good and evil. Little else is known about it, except that it supposedly can only be accessed if good and evil cooperate. However, the Source of All Evil was powerful enough to open the portal independently.
When the magic of the Charmed Ones and the Four Horsemen of the Apocalypse intertwined, it opened a portal to this plane and trapped Prue and War in the netherworld. However, they were still able to interact with the physical world, such as by moving the spirit board and through television.
In order to get Prue and War back, the sisters agreed to work with the Horsemen. However, the sisters later decided otherwise as they knew the Horseman would initate the apocalypse if the fourth horseman returned. Because of their selfless act, the plan of the the Source of All Evil failed and he opened the portal himself, vanquishing the other three horsemen. Piper and Phoebe then quickly cast a spell to retrieve Prue before the portal closed.