Most people who have not really thought about this topic would say the being known as Satan is God's opposite; one in control of hell. This is not accurate in the least.
To say that Satan is God's opposite implies that he has the same amount of power as God and equals Him out in a balance of power struggling for the control over human souls. No. He is not. He is in opposition to God but he is not the opposite of God. To say that he is in control of hell is giving him more authority than he has and says that he controls the situation. He does not. He is not in control of hell for one day he will be controlled in hell.
A. Who is Satan?
Satan is a supernatural being that has lost his way to God. Just like the human race he threw away his relationship with God. But unlike the human race he cannot get it back. This is why he is consistently and constantly trying to interfere in the human race. No one knows his real name but "Satan" means adversary. There are names that we have for him; the devil, Beelzebub, and others. See Names of Satan for more about who he is.
B. Where is Satan?
Well, that is an unanswerable question, for now. We know that one day he will be thrown into the lake of fire. We know that he has been thrown down from heaven. We know that he has traveled the earth and is called the prince and ruler of the world, prince of the air. Some would say that means he is still here on the earth (not in physical form and cannot be in physical form). But the good news is since he is a created being then he cannot be everywhere. This is how we know is also not omniscient. He does not know all your thoughts. See Existence of Satan for more about where he is and where he came from.
C. When did Satan come into existence?
Again, he is a created being. And since he is a created being he was not there "in the beginning"; meaning that he is not eternal and everlasting. He was created some time before the creation of the tree of knowledge of good and evil, otherwise there would be no "evil" to know. We know that there was a war in heaven and he was cast out. That, felt by most people, happened before creation of man. What we will agree on is that he will be held bound for eternity.
D. What does Satan do?
In essence, he is there to destroy the human race. To understand his reasoning is beyond human capability. He does not know your thoughts but he will try and influence you. He finds your weaknesses. He plays on your fears. He finds your worst temptations. Scripture says he like a lion waiting to devour his prey. But perhaps the best way to know what he does by knowing that he knows the word of God and tries to get you to doubt it. The very first thing he did to the human race was add doubt. Doubt is his biggest weapon. Atheists all doubt. Agnostics don't know what to believe. Believers are limited (but not destroyed) due to their doubt.
In the end Satan will try to control the world. He will possess a human that people call the anti-Christ. He will "unite" the world in a false sense of hope and peace. He will empower leaders from around the world filled with his ideas, many know this as the beast with ten horns. One person he will fill with so much information about his ideas that this person will become enamored with him ad become his personal speaker and worshiper, known as the false prophet. He will perform physical miracles that will cause people to believe in him as the Messiah, especially when will be resurrected. He will cause destruction and kill as many new believers as possible. And then he will be bound for 1000 years while the believers of God finally see the earth the way it was meant to be when a new heaven and a new earth will be presented to us. After that he will be released one more time but this time only believers who have no more doubt will run the earth and after his failure he will be thrown into the lake of fire.