Now, I've read the Hobbit. I've read the Lord of the Rings. But I've never read The Silmarillion all the way through. I've only ever read up to the song of creation, and what happened afterwards. Here's the basics.
Way back when, Eru decided to create some stuff. The first things he created were the Ainur, incredibly powerful people. However, during the creation of the world, one of the Ainur named Melkor decided that he knew better than Eru, and changed a few things. All through the history of this world, Melkor was working on destroying things.
Fast forward a few thousand years. Sauron, apprentice of Morgoth, apprentice of Melkor, is now a very powerful man. He's been taking care of things, even now that both Melkor and Morgoth have been destroyed. He appears to the elves, appearing as a fair character, and teaches them magic. He creates the one ring, and invests it with most of his malice, power, and evil. Now incredibly powerful, but at the same time with a crucial culnerability, he takes on the elves. Fortunately, he is defeated, despoiled of the one ring, and loses most of his physical form.
Sauron in all his Battle Gear.
I wonder how much the helmet weighs.
Much later, he is now ready to take on the world again. However, he really can't do much. His physical body is much to weak now to actually do anything, so he has to depend on others. The Nazgul, the nine men corrupted by the power of the One Ring, do his bidding. In addition, he corrupts Saruman, the most powerful wizard in the land, and has him create armies of orcs. Though he himself can do nothing, his armies can do quite a bit.
At the end, for all of his power and armies, Sauron is defeated. The One Ring is cast into Mount Doom, any remnant of a physical body is destroyed, and he is left as a powerless spirit, defeated and cast into the Void.
In the same way, Satan himself can do almost nothing. A mighty and powerful spirit of God, he aspired to be even greater, to become God himself. He essentially stated that he knew more than God, and that people should worship him. Thrust out of heaven, he will never have the privilege of having a body. He can do very little to us. However, he can tempt us to commit sin, to rebel against God. He can convince other people to see the world his way, and to act as he would like. We can withstand his temptations, though. Only when we are vulnerable and distracted does Satan try to thrust in some tentacles. He wants us to be miserable, as he is. I imagine that he knows he'll lose eventually, but he's determined to take as many of us as he can with him.