Stress is a normal part of life. Trauma is (basically) too much, too soon.
The following is a simplified explanation of how, ideally, the body reacts to a potential threat (real or imagined). The Autonomic Nervous System (ANS) has several functions, one being it responds to threat. There are two branches within the ANS, the Parasympathetic Nervous System and the Sympathetic Nervous System. The two function in balance with one another. In a relaxed state, the Parasympathetic Nervous System is primary. When stress occurs, the Sympathetic Nervous System is triggered and becomes primary, resulting in the body's natural defense of fight, flight or freeze. After the threat is no longer present, the system regulates and the Parasympathetic Nervous System becomes primary.
Trauma happens when the nervous system is overwhelmed and cannot take action (fight or flight) against a perceived threat to life or limb. Too much happens too fast. Trauma can be real or perceived, i.e., an ordinary non-threatening experience during childhood can be fine for one child and yet overwhelming for another child. Trauma can develop as a result of one overwhelming event (i.e., car accident) or from years of excessive stress and abuse. This is especially true in cases of neglect and abuse in early childhood.
Chronic stress and trauma cause a constrictive reaction in the body (dysregulation), which weakens the nervous system. Without self-regulation, the person remains highly activated, which is a defensive state, and their nervous system is unable to fully rest and recover. Without treatment, our ability to tolerate stress weakens as the internal chaos increases, causing physical, mental, emotional and/or spiritual suffering.