There are five words that are used in the New Testament to describe the final judgement of the unrighteous. The most frequent word in the New Testament that is rendered hell is gehenna. In this post, we will explore its use prior to the New Testament and then within the New Testament itself.
The word gehenna does not appear in the LXX (Greek Old Testament) or any Greek literature. It is the Greek form of the Aramaic gehinnam, which in turn goes back to the Hebrew ge hinnom. This originally denoted a valley lying to the south of Jerusalem, the valley of the son (or sons) of Hinnom. Child sacrifices were offered in this valley (2 Kings 16:3; 21:6). Josiah had it desecrated according to 2 Kings 23:10 and it will be the place of God’s judgment according to Jeremiah 7:32 and 19:6.
Jewish apocalyptic thought assumed this valley would become the hell of fire after the final judgment. In time gehenna became simply the place of punishment and so attracted the ideas about Hades. Gehenna thus became a temporary place of punishment until the final judgment. At about the end of the 1st century A.D. or the beginning of the 2nd century A.D., the doctrine of a fiery purgatory arose among the Jewish rabbis. All those in whose cases merit and guilt are equally balanced go to gehenna. There they are purified and, if they do penance, inherit paradise. 
In Jesus’ day, the valley was used as a burial place for criminals and for burning garbage.
There are twelve references to gehenna in the New Testament. All twelve of the references are used metaphorically as the place of fiery judgement. With the exception of James 3:6, which refers to the tongue being set on fire by gehenna, all the references are in the synoptic gospels (Matthew, Mark, Luke) as sayings of Jesus. 
For the New Testament, gehenna was a pre-existent entity (Matthew 25:41) and a fiery abyss (Matt. 13: 42, 50). It was the place of eschatological punishment after the last judgment, a punishment of eternal duration (Matt. 25:41, 46; 23:15, 33). Body and soul are judged in it (Mark 9:43ff). It was also to be distinguished from Hades which houses the souls of the dead before the last judgment. 
There are four other words in the New Testament that indicate the destiny of the unrighteous. In the next post, I will look at those words and then raise some questions as we try to correlate them.
1. New International Dictionary of New Testament Theology (4 Volume Set), Vol 2, 208.
2. James Leo Garrett, Systematic Theology: Biblical Historical and Evangelical (Vol 2), 787.
3. New International Dictionary of New Testament Theology (4 Volume Set), Vol 2, 208.