Greek hades derives from the same Indo-European root as English hell, which in pre-Christian polytheism was the gloomy place of the dead and often of torment. Gehenna is the inferno, a place of eternal destruction. Tartaros is the abyss, the lowest part of hell reserved for fallen angels.
The Greek words “Hades” and “Gehenna” are both translated into the word “hell,” though the concepts are dissimilar. Martin Luther, for example, translated the word “Hades” five times as the German word for “hell” (Hölle) (for example Matthew 16:18), and twice as “the dead,” twice as the “world of the dead,” and once as “his kingdom” (all in German). “Gehenna” was translated by Martin Luther eight times as “hell” (for example: Matthew 5:22,29,30; Matthew 18:9; Mark 9:43,45; and so on)and four times as “hellish.” In Norse mythology the underworld was a cold, monotonous place, which was commanded by the goddess Hel. The place was called Hel, too.
Newer translations of the Bible translate “Hades” or “Sheol” into the words “world of dead,” “underworld,” “grave,” “crypt” or similar, but still translate the word “Gehenna” into the word “hell.”
The word “Hades” of the New Testament is the Greek translation of the Hebrew word “Sheol” of the Old Testament (Acts 2:27; Psalms 16:10). What happens in Hades, or rather Sheol, Ecclesiastes tells us: “for in the Sheol, where you are going, there is neither working nor planning nor knowledge nor wisdom.” (Ecclesiastes 9,10) and “For the living know that they will die, but the dead know nothing; they have no further reward, and even the memory of them is forgotten.” (Ecclesiastes 9:5; see also Psalms 89:49; Psalms 139:8; Numbers 16:30). “The Lord brings death and makes alive; he brings down to the Sheol and raises up.” (1 Samuel 2:6).
Geenna (or Gehenna) was an earthly place used as a metaphor for the eternal destruction of evil. It comes from Hebrew and means “Gorge of Hinnom (Ge-Hinnom).” This gorge can still be visited today near Jerusalem. In the time of the Old Testament it was a place where children were sacrificed to the Ammonite god Molech (2 Kings 23:10).
That cultic practice was, according to the Old Testament, imitated by King Solomon in the 10th Century B.C.E. and under the leadership of king Manasseh in the 7th Century B.C.E. and in times of crisis until the time of exile of the Jews in Babylon (6th Century B.C.E.). The prophet Jeremiah, who condemned that cult strictly, called the valley the “gorge of killing”(Jeremiah 7:31-32; Jeremiah 19:5-9).
Gehenna became later a central garbage dump, to stop the practice of child sacrifice. At the turn of the 1st Century C.E. the gorge was used also to burn the dead bodies of criminals after their execution. The image of burning dead bodies probably inspired Jewish, and later Christian theologians to translate that place into the word “hell.”
The sea of fire after the last tribunal in Revelation 20:14 isn’t translated into the word “hell,” but sometimes gets the connotations of “hell.” In that sea of fire are thrown the beast, the devil, the false prophet, and Hell (Hades) itself, along with evil-doers, according to Revelation 20:12-15.
In 2 Peter 2:4 a deep, dark dungeon reserved for certain fallen angels is referred to as Tartarus and often translated as hell.