I read the entire entry in Wikipedia, but it seems kind of vague. I just don’t get what exactly it is. I know it makes someone feel uncomfortable feelings about a past event but how exactly do they respond? Is it the same as depression? Why does it say they feel nothing and are expressionless whereas for depression it says they feel sad?

Symptoms of intrusive memories may include:

Flashbacks, or reliving the traumatic event for minutes or even days at a time
Upsetting dreams about the traumatic event
Symptoms of avoidance and emotional numbing may include:

Trying to avoid thinking or talking about the traumatic event
Feeling emotionally numb
Avoiding activities you once enjoyed
Hopelessness about the future
Memory problems
Trouble concentrating
Difficulty maintaining close relationships
Symptoms of anxiety and increased emotional arousal may include:

Irritability or anger
Overwhelming guilt or shame
Self-destructive behavior, such as drinking too much
Trouble sleeping
Being easily startled or frightened
Hearing or seeing things that aren’t there