The PTSD or the post-traumatic stress disorder is easy to explain - this is a disorder that follows a traumatic incident. Your mind, filled with the horror and terror of the helplessness you felt during that particular incident, would overreact whenever anything triggers (or reminds you) about the scene. For example, a child is sexually abused by someone close in her or his family, but is unable to stop it or tell it to anyone. When this person grows up into an adult, he or she would have panic attacks every time he or she would be reminded of that incident. This could be a smell, a person who looks like the abuser, a particular word, a sound, a certain type of weather, a song and the like.