Trigger warnings: trauma, abuse, medical PTSD, armed forces service/combat PTSD
What do you think of when you think of PTSD—Post Traumatic Stress Disorder? For me, I think of the military veteran I once met in a mall, talking about his service dog, as well as the PTSD service dog he is training for Courageous Canines, a local organization that trains service dogs to help veterans who have PTSD, as well as for people with autism, and other medical conditions. I also think of many of my friends who have PTSD induced by medical events or abuse. These things—combat service, medical situations that may be life threatening, or past physical or sexual abuse, are all examples (but not inclusive) of traumatic situations that may lead to PTSD. Sometimes, PTSD is caused by a string of traumatic events (such as the case of military service or abuse), not just one single event. 
PTSD is a mental illness that causes a very strong reaction of fear, even long after the traumatic situation has ended—these symptoms must occur for at least 1 month following a traumatic event to be classified as PTSD. [1, 2] Post Traumatic Stress Disorder is classified as an anxiety disorder , and symptoms are different for everyone. Re-experiencing events vividly is common—this is often referred to as a flashback, but can also involve nightmares of the situation—because of this, people often avoid situations that are similar to those that cause these symptoms, known as “triggers”.  PTSD can cause mood changes, issues concentrating, being “on edge” , sleep problems, and feeling disconnected from people, and their own body and emotions.  PTSD can also affect children, in which cases they may act out traumatic situations frequently through play, have nightmares related to the event, or re-enact traumatic experiences.  People with PTSD may be re-shaped by traumatic events experiences, and frame thoughts about themselves and the world through the trauma they have experienced, for example, that they cannot trust anyone or that they are a bad person, experience persistent negative emotions like anger, fear, guilt or shame, and/or be unable to experience positive emotions, and experience disinterest in activities (as might be seen in or mistaken for depression). 
There are a variety of treatment methods used for PTSD, similar to other mental illnesses. Counselling or therapy, medication, and support groups of people who have similar experiences with PTSD, are all common types of treatment.  As well, having a strong network of family and friends to lean on, who have taken time to understand your situation to the best of their ability, can be helpful.  Treatment is individual for everyone, so the most important thing is to begin the conversation with your doctor or a mental health professional to start determining next steps to identifying and managing Post Traumatic Stress Disorder.
The Anxiety and Depression Association of America has a free screening tool, as well as an app for helping manage PTSD (from the US Department of Veterans Affairs). Also visit the ADAA to learn more about PTSD symptoms in-depth.
If you have Post Traumatic Stress Disorder, medical ID for PTSD can help identify treatments to others, or provide brief instructions for how to assist you if you are struggling. For instance, engraving Post Traumatic Stress Disorder – See Wallet Card can help those treating you find your care plan, enabling you to focus on the techniques you use to manage your PTSD.