The word gehenna is the Greek transliteration of the Hebrew ge-hinnom, meaning “Valley of [the sons of] Hinnom.”

This valley south of Jerusalem was where some of the ancient Israelites “passed children through the fire” (sacrificed their children) to the Canaanite god Molech (2 Chronicles 28:3; 33:6; Jeremiah 7:31; 19:2–6).

In later years, Gehenna continued to be an unclean place used for burning trash from the city of Jerusalem.

God so despised the false god Molech^[1]http://www.gotquestions.org/who-Molech.html  that He explicitly forbade the Israelites from having anything to do with him in Leviticus 18:20. He even warned them of the impending judgment He would send their way if the Jews didn’t keep their attention and worship directed toward Him. In another prophetic warning, God re-named the Valley of Hinnom as the Valley of Slaughter (Jeremiah 19).

But the Israelites didn’t listen, and evil kings of Judah such as Ahaz used the Valley of Hinnom for their demonic practices (2 Chronicles 28:3). To punish Judah, God brought Babylon against them, and that pagan nation carried out His judgment against Judah’s idolatry and rebellion.

It wasn’t until after 70 years of exile that the Jews were allowed back into Israel to rebuild. Upon their return the Valley of Slaughter was re-purposed from a place of infanticide to an ever-burning rubbish heap (2 Kings 23:10). Child sacrifice and other forms of idol-worship ceased in Israel.

Gehenna presented such a vivid image that Christ used it as a symbolic depiction of hell: a place of eternal torment and constant uncleanness, where the fires never ceased burning and the worms never stopped crawling (Matthew 10:28; Mark 9:47–48).

The Gehenna Valley was thus a place of burning sewage, burning flesh, and garbage. Maggots and worms^[3]http://www.gotquestions.org/worm-will-not-die.html crawled through the waste, and the smoke smelled strong and sickening (Isaiah 30:33). It was a place utterly filthy, disgusting and repulsive to the nose and eyes. Gehenna presented such a vivid image that Christ used it as a symbolic depiction of hell: a place of eternal torment and constant uncleanness, where the fires never ceased burning and the worms never stopped crawling (Matthew 10:28; Mark 9:47–48).

Because of Jesus’ symbolic use of Gehenna, the word gehenna is sometimes used as a synonym for hell. In fact, that’s how the Greek word is translated in Mark 9:47: “hell.” The occupants of the lake of fire/gehenna/hell are separated from God for all of eternity.

Source: http://www.gotquestions.org/Gehenna.html