The Dalek Asylum was a snowy and mountainous planet used by the Daleks as a prison and "dumping ground" for those among them who had malfunctioned, gone insane and/or become mentally scarred by battles. The Dalek Asylum was protected by a force field and a nanocloud, which converted beings that did manage to enter into Dalek puppets. The sane Daleks left their insane fellows in the Asylum rather than kill them because they epitomised the Dalek concept of beauty: pure hatred.
Even amongst the deranged Daleks, there was still an area described as "intensive care", which housed several Daleks who had survived conflicts with the Doctor on planets such as Spiridon, Kembel, Aridius, Vulcan, and Exxilon. (TV: Asylum of the Daleks)
The Alaska was able to breach the Dalek Asylum's force field and crashed into the planet. Junior Entertainment Manager Oswin Oswald was fully converted into a Dalek by the inmates, while the rest of the crew were converted into Dalek puppets by the Asylum's nanocloud. The Asylum was originally fully automated, but over the course of a year, Oswin was able to reduce the Asylum to a ruin by hacking into its systems.
A year after the Alaska crashed on the Asylum planet, the Parliament of the Daleks feared the insane Dalek inmates could escape. In order to destroy the planet and the Daleks inhabiting it, they kidnapped the Eleventh Doctor, Amy Pond, and Rory Williams, and sent them to the surface (by way of a gravity beam) in order to deactivate the planet's defences, as no Dalek was willing to infiltrate the Asylum themselves due to fear of the insane Daleks.
When the Great Intelligence invaded the Doctor's time stream on Trenzalore to turn all of his victories into defeats, the Dalek Asylum was one of the many places where the Doctor died. When Clara Oswald followed him into the time stream, it was revealed that "Oswin Oswald" was one of many versions of Clara scattered across the Doctor's history with the purpose of saving his life. (TV: The Name of the Doctor)