A nice easy question.
Sheol in Hebrew (translated ‘Hades’ in Greek New Testament) is simply the grave where “the dead know nothing” (Ecclesiastes 9:5)
Well the Valley of the Sons of Hinnom (Ge Ben-Hinnom, ‘Gehenna’ in the Greek New Testament) is a valley just outside the South East wall of Jerusalem. Today it is a pleasant park with a small cinema, but back in the days of Jeremiah it was the site of Tophet, a pagan shrine where children were sacrificed to the Ammonite god Molech (II Kings 23, 10). This pagan shrine was desecrated by the reforming king Josiah, but the name lived on as a symbol of evil, and was used by Jeremiah as the base of a prophecy about a future judgement day at Jerusalem. Jesus made use of the Gehenna prophecy many times in the Gospels, in Mark 9 connecting Gehenna back to the unnamed valley of the closing words of the book of Isaiah:
“And they shall go out and look on the dead bodies of the men who have rebelled against me. For their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh.” (Isaiah 66:24 ESV)
“Gehenna……. where their worm does not die and the fire is not quenched.’ (Mark 9:48 ESV)
Note that in neither Isaiah nor Mark 9:48 are the ‘worms’ souls of the wicked, but simply real worms with so many bodies that they continue feasting for a long time.