Who is Gaia?
Gaia, in Greek mythology, is the primordial mother of all things. It is from her that the gods were born as well as the Titans. Her counterpart was Uranus (the sky), whom she created as well. We can read about her in Heiod's Theogony. From a more modern stand point, she is considered mother earth.
Since you tagged, science in the key words, I will also address the Gaia principle in the scientific community.
Some modern scientists have taken this name from Greek mythology and created a scientific hypothesis. These scientists call this the Gaia hypothesis. They posit that all things (organisms and their surroundings) are interrelated and form complex unit. They posit that this complex unit has all that is necessary for life. More specifically, this theory argues that this complex unit seeks for the optimal conditions for the sustenance of life.
As you can imagine, many scientists reject these theories.