What is IT from A Wrinkle In Time?

In A Wrinkle In Time, IT is a disembodied brain, that is to say a brain without a body. But it's an incredibly powerful brain, all the same, which allows it to exercise absolute power over the inhabitants of the planet Camazotz. This pulsing, quivering blob of gray matter exerts an almost hypnotic effect over everyone. It speaks through vibrations of thought which allows it to control other people's minds. That's how IT comes to be so powerful.
IT likes to present itself as benign, as saving everyone the pain and trouble of thought and decision. But this is only so it can continue to control people's minds and lives. It may seem a tempting prospect to have someone—or something, in this case—do everything for you in life, but in actual fact, it would turn you into some kind of robot, without any kind of freedom or autonomy.
And that's exactly what happens to the inhabitants of Camazotz. They may appear human, but in reality they're little more than automata, their every movement controlled by a powerful, malevolent brain that punishes anyone who somehow manages to break free from their programming.