Gaia theory is the notion that systems are able to organise their environment in order to ensure their own survival - even if these systems are inanimate (like a planet).
The hypothesis was formulated by the chemist James Lovelock and co-developed by the microbiologist Lynn Margulis in the 1970s.
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The Breathing Earth is a nice visualisation of the seasonal cycles of the earth from outer space - showing how the earth and it's living ecosystem "breathes". A nice portrait of Gaia - arthtoparis.org 
The hypothesis was initially criticized for being teleological and contradicting principles of natural selection, but later refinements resulted in ideas framed by the Gaia hypothesis being used in fields such as Earth system science, Biogeochemistry, systems ecology, and the emerging subject of geophysiology.
Nevertheless, the Gaia hypothesis continues to attract criticism, and today many scientists consider it to be only weakly supported by, or at odds with, the available evidence. In 2006, the Geological Society of London awarded Lovelock the Wollaston Medal largely for his work on the Gaia theory.
# See also
A nice visualisation of the seasonal cycles of the earth from outer space - showing how the earth and it's living ecosystem "breathes". A nice portrait of Gaia