Hell was an Old-English word that, at the time when the fine classic King James Version was translated in 1611, meant simply the grave, or a hole in the ground. The word hell was used by the KJV translators, and many others ever since, for a number of different original Hebrew or Greek words of The Bible. Not all mean a place of fire.
Sheol
The Hebrew word pronounced she-ole meant the grave (see Sheol) e.g Psalm 16:10 from various translations:
King James Version: "For thou wilt not leave my soul in hell; neither wilt thou suffer thine Holy One to see corruption."
Revised Standard Version: "For Thou dost not give me up to Sheol, or let Thy Godly One see the pit."
New International Version: "because You will not abandon me to the grave, nor will You let your Holy One see decay"
Hades
The Greek word pronounced haw-dace, was the equivalent of the Hebrew word sheol - it too meant the grave.
"For thou wilt not abandon my soul to Hades, nor let thy Holy One see corruption." (Acts 2:27 RSV)
"he foresaw and spoke of the resurrection of the Christ, that He was not abandoned to Hades, nor did His flesh see corruption." (Acts 2:31 RSV)
As the Scriptures above describe, Jesus Christ was in "hell," the grave in The Garden Tomb, for three days before His resurrection.
Gehenna
The Hinnom Valley is a deep, narrow ravine located in Jerusalem, running south from the Jaffa Gate on the west side of the Old City, then eastward along the south side of Mount Zion (seen in the top of the photograph with the Hinnom Valley below it) until it meets the Kidron Valley which separates the Temple Mount from the Mount of Olives on the east side of the city. It is named from a certain "son of Hinnom" who apparently owned or had some significant association with the valley at a time prior to Joshua 15:8.
The Valley of Hinnom had a very horrendous history in ancient times. It was used as a place where the pagan worshipers did all sorts of vile and wicked things - including burning children alive as sacrifices to the idols Moloch and Baal. One section of the valley was called Tophet, or the "fire-stove," where the children were slaughtered (2 Kings 23:10). It was a place of tremendous evil for many years.
After their return from the Babylonian exile (see Why Babylon?), the Jews turned the Hinnom Valley into the city dump where garbage and anything deemed unclean (including the bodies of executed criminals) was incinerated. For that purpose, a fire was kept constantly burning there. Even though it was no longer used for evil worship, with all the filth and thick smoke it remained a very dark and dreary place.
The Hebrew name Hinnom when translated into Greek is gehenna. By the time of Jesus Christ, the deep, constantly-burning Valley of Hinnom was also known as the Valley of Gehenna, or Hell, and had taken on a popular image as the place "down there" where the wicked would eventually be cast into the flames for destruction.
Jesus also figuratively used the original word Gehenna in His dire warning to unrepentant sinners, but He obviously wasn't referring to the fires of the Valley of Hinnom - He was talking about the future lake of fire that will be used to destroy the enemies of God (Revelation 19:20, 20:13-15).
The Lake Of Fire - The Real "Hell"
"And I saw the beast and the kings of the earth with their armies gathered to make war against him who sits upon the horse and against his army. And the beast (see The Antichrist and The Lawless One) was captured, and with it the false prophet (see The Great False Prophet) who in its presence had worked the signs by which he deceived those who had received the mark of the beast and those who worshiped its image. These two were thrown alive into the lake of fire that burns with sulphur." (Revelation 19:19-20 RSV)
"Then I saw a great white throne and him who sat upon it; from his presence earth and sky fled away, and no place was found for them (see The Last Day). And I saw the dead, great and small, standing before the throne, and books were opened. Also another book was opened, which is the book of life. And the dead were judged by what was written in the books, by what they had done. And the sea gave up the dead in it, Death and Hades gave up the dead in them, and all were judged by what they had done. Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire; and if any one's name was not found written in the book of life, he was thrown into the lake of fire." (Revelation 20:11-15 RSV)
Fact Finder: Will the surface of the earth one day be "flooded" with fire? 2 Peter 3:5-7,10-11