There are two primary forms of stressors (that which evokes a stress response), psychosocial stressor and biogenic stressors. 

Psychosocial Stressors involve a cognitive interpretation of the event and the meanings that are assigned (Lazarus, 1966).  The illustration to the right exemplifies one stressor, with two interpretations resulting in a child that is apparently calm about getting the shot and the man upset.

The biogenic stressor does not need a cognitive interpretation.  The nature of the biogenic stressor itself is enough to produce a stimulative reaction.   This is called a sympathomimetic characteristic. In the second illustration, the effect of cold (biogenic stressor) exert a sympathomimetic effect causing physiological arousal without the necessity of interpretation. Other examples are caffeine, exercise and heat.

Good Stress and Bad Stress

There are two types of stress. Eustress is a positive kind of stress that lets you thrive and keeps you full of energy and able to cope. The other type of stress is distress, which makes you feel overwhelmed and drained of energy.  First, it helps to know that stress is just our bodies' reaction to a new situation. Our bodies react in the same way, whether the situation is welcome or unwelcome. Our hearts beat faster, our bodies produce adrenaline, we breathe more rapidly, we are more alert, and we may perspire more when involved in a happy event like marriage, or a threatening event like an exam. All of these responses can be good for you when followed by periods of relaxation. They can give you the "competitive edge" in sports, keep you alert and concentrated for a test, or can literally save your life in an emergency. But, when your body is constantly wound up and can't relax, these same responses can lead to increased blood pressure, anxiety attacks, and a wide range of physical disorders. The important point is that, physiologically, your body cannot differentiate between the sources of the stress, whether good or bad, and reacts the same in both situations. It is easy to discount any impact of  stress when the event is positive, but nonetheless the body still responds.  Consider a person getting married, changing residences, and their grandparent is taken ill. It is easy for us to just focus on the grandparent being ill, but the body has been sustaining an ongoing stress response throughout the positive events as well.

It is important to understand that once a stress response is activated whether positive or negative, whether the stressor is psychosocial or biogenic, there exists a target-organ.  For example, if we drink coffee our body responds with arousal, our heart rate increases, our brain releases epinephrine and norepinephrine to produce the rush-like sensation. In moderation, this is fine, however if the person drinks 10-15 cups a day of coffee, there becomes the risk of cardiac conduction abnormalities, or damage to a target organ like the heart. Even if the person's interpretation of their coffee intake is no big deal, damage can still incur.  Similarly, a stress response will decrease appetite.  This is seen as a survival mechanism in the temporarily shutting down of this biological need to increase resources to deal with the stressor. If the stressor is ongoing, stomach problems may result.