Very cool question. And it is a question that has puzzled scientists for many years.
The simple answer, I would give, is the ability self awareness, of knowing that you exist, knowing where you exist and being able to comprehend the affects of your own actions on others and on your self. Consciousness is when you understand yourself and the environment you live in and you can learn from your own experiences.
But there is so much research on the topic of consciousness as well as on explaining why it is very difficult to explain consciousness 🙂 I hope some day some one can really give a complete answer to that question 🙂