This clip from the Big Bang Theory explains what homeostasis is. Homeostasis is defined as: the tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes. Every living organism must maintain a stable internal environment to be able to react to the environment and stay alive. For example if an organism can't keep a stable body temperature they will freeze to death.
'Body Atlas' shows where emotions hit the hardest (note: not a Flir cam showing actual heat signatures, but rather a rendering of test subjects' indications of feeling high vs. low response in the body. Still freaking cool, though.) via The Verge and PSFK