The Greek word that we translate as "hell" is Gehenna. It comes from the Hebrew word for the Valley of Hinnom, or Ben-Hinnom, a valley that lies outside Jerusalem (see Joshua 15:8). It was infamous for being the place where children had been sacrificed by fire in pagan rituals (see 2 Chronicles 28:3). There is no clear proof that it was ever a garbage dump where refuse was burned. Still, over time Gehenna also became the name of the place where sinners were punished after death.
In the New Testament, the suffering of hell is mostly pictured as fire (see Mark 9:43) but also as darkness (see Matthew 25:30; 2 Peter 2:17) and as destruction and exclusion from the presence of the Lord (see 2 Thessalonians 1:9; Matthew 7:21-23). The point is less to describe hell in detail than to suggest it is a place of torment.
It can be safely assumed from Scripture that hell is just as everlasting as heaven (see Matthew 25:46). There is no talk anywhere in the New Testament of people ever leaving hell. The longest and most widely held view is that those in hell experience torment for eternity, and there remain strong arguments for this view. Some evangelicals, including John Stott, believe there is a biblical case to be made for annihilationism—the theory that at some point those in hell experience "the second death" (Revelation 20), or what is also called eternal destruction. That is, their existence simply ceases, and they suffer eternal consequences in the sense that not the punishment but the consequence lasts forever. Annihilationism remains a minority view, but in either case, the consequences of rejecting God last for eternity.
The doctrine of hell, like most others, comes packaged with other ideas that can't be separated from it. The most important is the Last Judgment.
The idea of God as "Judge of all the earth" appears early in the Bible (Genesis 18:25). God "will judge the world with justice and rule the nations with fairness" (Psalm 9:8). His judgment includes both punishment for the wicked and reward for the faithful:
I, the LORD, love justice.
I hate robbery and wrongdoing.
I will faithfully reward my people for their suffering
and make an everlasting covenant with them (Isaiah 61:8).
The Lord will wash the filth from beautiful Zion
and cleanse Jerusalem of its bloodstains
with the hot breath of fiery judgment (Isaiah 4:4).
Many people think the Old Testament is the testament of judgment and the New Testament the testament of grace. That distinction is true in only a limited sense; in fact, the New Testament intensifies the Old Testament ideas of judgment. Similarly, many believe that in the Old Testament we witness a judging God but in the New Testament a merciful Jesus. In the New Testament, however, judgment now becomes associated closely with Jesus himself. The fact that in the New Testament the judge has become Jesus is a crucial factor to note, for it will affect how we understand many of the questions that swirl around hell and judgment.
The Last Judgment is a major theme in the parables of Jesus (see Matthew 13:24-30, 36-43, 47-50; 21:33-41; 22:1-14; 25:1-13, 31-46, among others). In the Gospel of John, in which Jesus talks so much about God's love, Jesus also says that he acts in God's stead in this capacity: God "has given the Son absolute authority to judge" ( John 5:22) and, "I judge as God tells me. Therefore, my judgment is just, because I carry out the will of the one who sent me, not my own will" (John 5:30).