Stressors are agents that cause stress. Stressors can be divided into two broad categories:
External
Internal

External Stressors
External stressors are the sources of stress that we are aware of around us. These stressors are things that create a situation of perceived threat in our minds and bodies. Over the last few years a lot of research has been done on external stressors. These stressors can affect us in various settings- at work, at home, while driving and in a social setting. We are all free sprits in Nature. Anything that constraints our freedom of expression, thought or action creates a situation of stress that out bodies and our minds would like to change. It results in a feeling of unhappiness and discontent. For example, the same home can be a happy place or a stressful place. If things do not happen according to our desires at home, our wishes are unfulfilled and we feel stressed. In the workplace, our expectation of our work, our employers, our colleagues and our own commitments, if unfulfilled, create a situation that our mind perceives as a threat. We find ourselves helpless and unable to change the situation. Our free spirit is stilted. We feel the pressure. This is stress.

Internal Stressors
Internal stressors are the stressors inside us. These are stressor that have been in our minds and bodies for many years of our lives. These stressors exist in the form of our genetic loading. They are also restrictions that have been imposed on us by our parents, our teachers and various authority figures that have taught us what to do and more importantly, what not to do. Such internal stressors remain in our bodies, unknown to us, in the form of emotions. According to recent researches done in the United States, such emotions exist in our bodies in the form of neurochemicals called neuropeptides. Neuropeptides are laid out in our cell systems in such a manner that the patterns are hard to delete. Such patterns, when laid out with our emotional experiences, persist for our lifetime. In this form, stress is stored in our bodies. These stressed or negative emotions or neuropeptides are organized in such a way as to affect our perception, our thinking and our behaviour. These stresses become chronic.

The interaction of the external and internal stressors and the internal stresses creates what we call stress. The internal stressors mentioned above, when interacting with the external stressors (environment), create arousal in our body systems. The internal stressors then become stress.