This is possibly the biggest mystery of the human mind - what in the brain creates the experience of consciousness?
We're getting ever closer to an answer. One group of scientists have even found and "on/off" switch in the brain that, when stimulated, causes the patient to completely lose consciousness, but regains it the moment stimulation ceases. However, that in itself doesn't explain what it is.
Some think that it is the result of countless complex interactions in the brain, some think that there is a specific point at which it all "comes together" and some have even suggested that it might be a quantum effect. The truth is that we simply don't know.
Think about, for example, the moment your alarm goes off in the morning. What is the difference between the moment that you're asleep, and the moment you wake? What is it about taking a heavy blow to the head that stops the mysterious experience of consciousness? Is it even the same experience for everyone?
For all we know, it's all an illusion and you're just a brain in a jar.