Angels, like man are eternal, created beings. The difference is, God created man in His own image, which is a spirit, and in order to properly function in a physical world, man was clothed in a physical body. Since the fall of man, through sin, that body is subject to physical death, which angels are not. With the death of the flesh, the spirit continues to exist, either in heaven or in hell. Those in hell will be there until the final White Throne Judgment, which is known as the "second death" or complete separation from God. Many are under the impression that Satan, the Devil, or whatever you want to call him is headquartered in hell, when actually the bible states that Hell is to be the place of his torment, not his domain, and that he right now roams about the earth trying to influence as many humans as he can to do his bidding, by rebelling against God through every sin imaginable. So far, it's working.