Who is Gaia?
In Greek mythology, Gaia is the personification of the Earth and one of the Greek primordial deities.
The Gaia Hypothesis was developed by James Lovelock and Lynn Margulis in the 60's. The Earth is a self-regulating complex system involving the biosphere, the atmosphere, the hydrospheres and the pedosphere, tightly coupled as an evolving system.
The hypothesis contends that this system as a whole, called Gaia, seeks a physical and chemical environment optimal for contemporary life.
From a concrete or physical point of view (what is it?):
It is the thin spherical layer of earth and water that exists between the incandescent interior of the planet and the upper atmosphere that surrounds it.
From a biological point of view (who is it?):
It is the interactive living organisms that have inhabited it for over four billion years.
The combination of that What and that Who and the way in which one continuously affects the other, is what has been baptized as Gaia.
The Revenge of Gaia: Why the Earth Is Fighting Back - and How We Can Still Save Humanity. London; 2006