Raa/others, basically, as Stealth said, according to Christian tradition, Satan is a fallen angel. He is not equally powerful to God; in fact, he is doomed to be destroyed at the end of time. It's kind of hard to explain about such matters, or even to put them into words, since a lot of it is more tradition than actual dogma. Suffice it to say that the Devil is the representation of the evil that is present in the world, and this evil will be finally defeated at the end of time. Also, regarding victories over evil, Christ's death and resurrection represents a great victory, since he conquered death and opened the way to eternal life.
As for the whole existence of Hell, according to Catholic teaching (and to most people in general, but that's beside the point) all human beings have free will. We are given the free choice to act for good or evil. Being good people brings us closer to God, while committing evil thoughts/actions turns us away. Either way, the choice is ours to make. Yes, God is all-forgiving, but we have to actually want forgiveness. Hell exists for those who have truly turned away from God, who are non-repentant. It is true that God is omniscient, but he still gives us the choice to follow him or not. Otherwise, we would be nothing more than slaves. In the Catholic view, humanity represents the pinnacle of creation and was made "in the image and likeness of God." We are not God's slaves; we are his children. Just as any parent must allow their children to learn from their own mistakes and to live out their lives, so God does with us.
Grey Wolf, I don't know where you've been looking, but Hell at least is mentioned a great deal in the modern Catholic faith. As for the Devil, try not to think of the horned, pitchfork-carrying, pointy-tailed Halloween representation. The Devil, instead of really being a certain persona, is the image of the temptation that we all face, the part of us that wants to turn away from God. As for Revelation, I think that a lot of people could look at it and believe that it's not supposed to literally portray the end of time. I believe, and the Catholic Church teaches, that it is an allegory or parable to the end of the world.