The Return of the King
Sauron is your average power-mad, minion-employing monster. Think the Emperor, a.k.a. Senator Palpatine, in Star Wars. Or Voldemort in Harry Potter. Or, as much as we love this guy in X-Men: First Class, Magneto in X-Men.
Like these other super-villains, Sauron wants to conquer the world and remake it according to his own wishes. He wants utter control. But unlike these guys, Sauron has basically no character development. We know what ideas and prejudices drive Magneto and Voldemort, but Sauron is about as flat and two-dimensional a character as you could possibly find. All you truly have to know about him is, he's Bad, and he has to be brought down.
But we do want to mention one important thing about what Sauron can and cannot do. Tolkien emphasizes that Sauron is great at manipulation. When Frodo talks about Saruman at the end of The Return of the King, he says, "Saruman was doing [Sauron's] work all the time, even when he thought he was working for himself" (6.8.213). In other words, Sauron is so good at twisting people's thoughts and intentions that he could even trick other villains.
Still, while Sauron is good at tricking people into doing things he wants them to do, he can't just make stuff out of the blue. Frodo comments:
The Shadow that bred [the orcs] can only mock, it cannot make: not real things of its own. I don't think it gave life to the orcs, it only ruined them and twisted them; and if they are to live at all, they have to live like other living creatures." (6.1.108)
So, even while Sauron is pretending that he is the new, ultimate power on Middle-earth, he still has limits. He can twist and distort things that already exist, but he cannot make "real things of [his] own." Sauron cannot create life; his power is just to damage the life that's already there. Frodo doesn't clarify what power he thinks does create life in Middle-earth, but we get into some of the religious symbolism in The Return of the King in our "Symbols, Imagery, Allegory" section.