When people who have rejected Jesus die, they go to Hades to await the last judgment. "Hades" is the Greek equivalent of the Hebrew word "Sheol", which means the dwelling place of the dead. Because they have not experienced the first resurrection and are spiritually dead, Hades is a shadowy existence. The souls of the wicked go there, but they have no life, because their spirits are dead.
Jesus holds the key of death and of Hades (Rev 1:18). At the end of the age, Hades will give up its dead prior to the last judgment.
The sea gave up the dead that were in it, and death and Hades gave up the dead that were in them, and each person was judged according to what he had done. (Rev 20:13).
Hades is a place of transition, a temporary dwelling place for those who have rejected Christ. At death they go to Hades to await the judgment. After the last judgment Hades will be destroyed, because its purpose is complete (Rev 20:14).
The only New Testament description of Hades is in the parable of the Rich Man and Lazarus (Luke 16:19-31). The rich man went to Hades when he died, while Lazarus went to Paradise (v. 22). . The events described take place while life is still going on upon the earth, so they are prior to the second coming (v. 27). This confirms that Hades is to the unrighteous, what Paradise is for the righteous.
Jesus parable does not tell us much about what Hades is like and we must be careful about reading too much into a parable. What we can be certain is that Hades is not a pleasant place and the rich man did not want to be there.
Another word, Tartarus, is only mentioned in 2 Peter 2:4.
For if God did not spare angels when they sinned, but sent them to hell (tartarus) putting them into gloomy dungeons to be held for judgement;
Tartarus is a place in which the angels who sinned are confined, while waiting for judgment. It is the angelic equivalent of Hades. It may be the same place as the “abyss” mentioned in Revelation 9:11.
The Bible does not tell us where Paradise, Hades or Tartarus are located. They exist somewhere in the spiritual dimension of universe.