What exactly is Fire?

The question in the title daunted me as a kid. What misguided me more was this:

After watching the anime Avatar, I was under the impression that just like Air, Water and Earth, Fire also must be made up matter. But then natural questions that came to my mind were - 1) What is the state of fire: Solid, Liquid or Gas? 2) What is the chemical composition of fire? 3) Why do flames emit light?

In this video I have tried to answer all these questions. Do let me know your thoughts over it.