It takes your worst experiences and constantly haunts you with them. My PTSD stems from Vietnam. I served in Vietnam for only 12 months but that evidently was enough. I had several experiences that are repeated in my mind almost everyday. Each one is just as vivid as if it were happening right now.
Flashbacks are vivid visual memories.
Most of my flashbacks are triggered by the smell of burning leaves and wood. Artillery rounds, bombs, and search and destroy missions all caused fires in Vietnam. So did helicopters and planes that were shot down. It seemed like everyday in Vietnam I saw at least one of the above happening. Nightmares are just that, nightmares.