Broadly speaking, there are two types of trauma, complex trauma and single incident trauma.
Complex trauma involves multiple traumas experienced across an extended period of time. Most often the traumatic events involve another person (or people). For example, physical, sexual or emotional abuse, neglect, community violence or war.
Single incident trauma involves witnessing or experiencing a “one-off” event such as a car accident, a natural disaster or a sexual assault.
When people experience or witness a traumatic event or repeated events, they can sometimes develop symptoms of post-traumatic stress or meet the criteria for a diagnosis of Acute Stress Disorder, Post-Traumatic Stress Disorder (PTSD) or Complex PTSD. Symptoms of post-traumatic stress may also arise when there is a threat that these events will happen, even if they do not actually end up happening.
You don’t have to have directly witnessed the event to have a post-traumatic stress response; it can also be through learning that a close family member or friend was exposed to trauma. Repeated or extreme indirect exposure to trauma can also lead to post-traumatic stress symptoms, for example, paramedics and other medical staff repeatedly witnessing horrific injury or death.