Hell is one of the more controversial subjects in Christianity. A lot of non-Christians use this as an attacking point (I've heard quite a few say that they could never believe in a God who would send people to an eternal torment like Hell, so we never even get into the evidence for God because they can't get past this starting point). Complicating this is the different descriptions of Hell contained in the Bible. Sometimes it is referred to as a place of eternal fire and torment. Other times it appears to be a place of destruction or separation from God. So what exactly is Hell?
Anyone who claims to have an obvious answer to this question probably doesn't have a good enough understanding of the scriptures. After all, this is a tough question, and we cannot be afraid to admit that it's tough. But for what its worth I'd like to put my two cents in and offer up my personal interpretation. By no means do I claim that this is 100% the "right" answer. But hopefully it will give you some food for thought, and I believe it is completely consistent with the scriptures.
When analyzing Hell you have to remember two things. First, the name "Hell" is actually our English translation of the word "Gehenna." Gehenna was a real place, not just some spiritual realm. Gehenna was the name of a burning trash dump. People who practiced paganism used to throw their children into the fire as a sacrifice to Molech, a pagan "god". So when Jesus said that people would be cast into "Gehenna", this would have conjured up a very specific image in the minds of His contemporary listeners. They would immediately have thought of themselves being thrown into this burning trash pit for all eternity. Now did Jesus literally mean that people who rejected God would be thrown into this earthly fire pit? No, of course not. It's a metaphor, designed to generate a particular image in people's minds. What image is that? Is it necessary to conclude that Jesus was telling people that Hell was a place of literal fire? I don't think so. My personal feeling (especially when we read these Biblical passages in conjunction with those discussing separation from God) is that Jesus was merely trying to convey that compared to the option of Heaven, Hell is a VERY undesirable experience.
The second important thing you need to remember is that many times the Bible ascribes things to God for which He is only INDIRECTLY responsible. In other words, it will say God did something, but what it really means is that God allowed that thing to happen. After all, God is in control of everything, so He could stop anything from happening. So when something happens, by necessity this means that God allowed it to happen.
In my opinion, this is precisely what is happening when the Bible says God casts people into Hell. He has the power to stop it, but He allows it to happen. Therefore the Bible attributes it to Him. Let me explain in more detail.
God is perfect. Heaven is essentially spending eternity with God. But because God is perfect, we can only spend eternity with Him if we too are perfect. Of course, we are far from perfect, which means that left to our own devices we cannot spend eternity with God. Hell is separation from God. Without divine intervention, Hell is the default destination for all souls after death. So it isn't that God makes an affirmative decision to send people to Hell. That is where we are headed anyway. However, God intervenes to allow us to thwart this default destiny and get into Heaven with Him. He gives us a way to be declared perfect in spite of our flaws. We can do this through faith in Christ, allowing our sins to be transferred to Christ so that He paid the price for them on the cross.
You also need to remember that Heaven is not only being in God's presence for eternity, but it also means submitting to God eternally. For believers, this prospect gives us joy. But someone who spends his or her life rejecting God, and certainly not submitting to Him, would have no interest at all in submitting to Him for eternity. Given the choice between Heaven and Hell, non-believers would probably choose Hell. At least there they could continue to be their own masters instead of having to submit. So it's not necessarily true that Hell is a place of literal flaming torment. But the only way to compare the enormous gap between the experiences of Heaven and Hell is to compare Hell to Gehenna. The difference between our normal life and burning in Gehenna captures the image of the difference between the ecstasy of Heaven and existence in Hell. But in order to appreciate that ecstasy you have to love God. If you don't love God, Heaven actually may be even a worse fate for you than Hell.
So when people say that God punishes people eternally simply because they don't believe in Jesus, they really are missing the point. It's not that God punishes them for unbelief. They are headed for Hell anyway. God actually saves people by offering them a way out of their predicament. Hell is the default. Heaven is the result of God's action.
For what it's worth, that's my opinion. Take it or leave it. It may be completely wrong, and there are certainly better theologians than me who may tell me I've got it all wrong. But if this brief explanation gives you some food for thought, then it has accomplished its goal. God bless.