Most English Bibles translate three different Greek words as “Hell,” but these terms do not all mean the same thing.
The word “Hades” (ᾅδης) is the term that the Greek Septuagint translation of the Old Testament normally used to translate the Hebrew term “Sheol” (שׁאול), which refers to the abode of the dead, whether righteous or unrighteous, prior to Christ’s Resurrection. The Greek term itself is taken from Greek Mythology, because the concept of Hades and Sheol were roughly equivalent. The parable of the Rich Man and Lazarus (Luke 16:19-31) shows us how Hades/Sheol was understood by Christ. Both righteous and unrighteous men were understood to go there, but they did not experience the same thing.
The word “Tartarus” (τάρταρος) is used only once in the New Testament, in 2 Peter 2:4. This word likewise comes from Greek mythology, where it refers to a place of torment for the wicked.
The term “Gehenna” (γέεννα) is found very frequently in the New Testament, but is not found in the Old Testament, though the idea of a final place of torment for the wicked certainly is (e.g. Isaiah 66:24). We also do not find the word in Josephus. Philo likewise does not use this word, but he does use the word “Tartarus.”
The term comes from an association with the Valley of Hinnom — but not the one usually repeated. The common explanation is that the Valley of Hinnom (which is on the southern edge of the old city of Jerusalem) served as the city garbage dump, and that there was a perpetual fire there to burn garbage. This explanation originated from Rabbi David Kimchi’s explanation that dates to around 1200 a.d., but this explanation is not supported by either archaeological evidence, or literary evidence from before or after that time (see The Myth of the Burning Garbage Dump of Gehenna).
The reason that the Valley of Hinnom became associated with the place of eternal torment is that this was a location in which child sacrifice was practiced (2 Kings 23:10; Isaiah 30:33; Jeremiah 7:32; Jeremiah 19:6).
Joachim Jeremias, in the entry for the word “Gehenna” (γέεννα) in the Theological Dictionary of the New Testament explains:
“This name [Gehenna] was given to the Wadi er-rababi, in South Jerusalem, which later acquired a bad reputation because sacrifices were offered in it to Moloch in the days of Ahaz and Manasseh (2 King 16:3; 21:6). The threats of judgment uttered over this sinister valley in Jer. 7:32; 19:6, c.f. Isaiah 31:9; 66:24, are the reason why the Valley of Hinnom came to be equated with the hell of the last judgment….” (The Theological Dictionary of the New Testament, Volume 1, ed. Gerhard Kittel (Grand Rapids, MI: Eerdmans Publishing, 1964-1976), p.657).
No one is currently in Gehenna. The wicked go to Hades when they die, and experience a foretaste of the judgment that awaits them. They experience this apart from their bodies. At the Resurrection, they will be raised with their bodies, and will experience what is called “the second death,” in Gehenna:
“Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, and shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation” (John 5:28-29).
“And death and hell [hades] were cast into the lake of fire. This is the second death” (Revelation 20:14).
“But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death” (Revelation 21:8).
So when we think of Hell, we are generally thinking of what the term “Gehenna” is referring to, and that term is equivalent to the terms “Tartarus,” and “Lake of fire.”
The saints, after Christ’s Resurrection are with Him in Paradise. If one dies in a state of sanctity they do not go to Hades. However, if someone dies in a state of repentance, but without having had a chance to bring forth all the fruits of repentance, we believe that they are not ready to enter immediately into the presence of God, but that at some point, through the prayers of the Church, they will be. They are given some period of time by God to grow in grace. They also experience a foretaste of what awaits them, before they actually enter into the presence of God.