All perfect praise be to The Lord of The Worlds; and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Hell is an eternal abode for the wretched who died while they are unbelievers. Allah, The Almighty, Said (What means): "Verily God has cursed the Unbelievers and prepared for them a Blazing Fire,- To dwell therein for ever: no protector will they find, nor helper." {Al-Ahzab, 64-65}. He, The Exalted, Said (What means): "Their wish will be to get out of the Fire, but never will they get out therefrom: their penalty will be one that endures." {Al-Mai`da/37}.
On the other hand, those who died while they are believers, but were admitted to Hell, shall be tortured according to their sins, then they will be taken out by The Grace and Mercy of Allah, The Almighty, and the intercession of His Messenger (PBUH).
The Prophet (PBUH) said: "When the people of Paradise will enter Paradise and the people of Hell will go to Hell, Allah will order those who have had faith equal to the weight of a grain of mustard seed to be taken out from Hell. " {Agreed upon}.
Abu Al-Hasan Al-Ash`ari (May Allah be pleased with him) said: "The Mo`tazilah and the Khawarij said about the wicked-sinners amongst the Muslims," Whoever of them enters Hell shall never get out;" whereas, the People of Sunnah and righteousness said: "Allah Shall Take the people of the Qibla (Those who pray in the direction of Ka`ba (Mecca) out of Hell and shall never make them dwell therein eternally." {Maqalat Al-Islamieen, vol.1/pp.474}. And Allah Knows Best.