General Adaptation Syndrome
Don't Let It Get You Down
We All Have It

Stress is how the body reacts to a stressor (something or reason that causes stress). There are acute stressors, which are a short term stress that can effect you greatly or regularly; and there are chronic stressors, which are a long term stress that can also effect you greatly or regularly. Acute stressors can make the way you deal with stress easier.

Alarm is the first stage. When the threat or stressor is identified or realized, the body's stress response is a state of alarm. During this stage adrenaline will be produced in order to bring about the fight-or-flight response. There is also some activation of the HPA axis, producing cortisol.

Resistance is the second stage. If the stressor persists, it becomes necessary to attempt some means of coping with the stress. Although the body begins to try to adapt to the strains or demands of the environment, the body cannot keep this up indefinitely, so its resources are gradually depleted.

Exhaustion is the third and final stage in the GAS model. At this point, all of the body's resources are eventually depleted and the body is unable to maintain normal function. At this point the initial autonomic nervous system symptoms may reappear (sweating, raised heart rate etc.). If stage three is extended, long term damage may result as the capacity of glands, especially the adrenal gland, and the immune system is exhausted and function is impaired resulting in decompensation.

The result can manifest itself in obvious illnesses such as ulcers, depression, diabetes, trouble with the digestive system or even cardiovascular problems, along with other mental illnesses.

Reference: http://en.wikipedia.org/wiki/General_adaptation_syndrome#General_Adaptation_Syndrome