I will try and explain it for you.
When we go into combat, or experience death and the horrors of war, we never get the chance to process it due to life in a combat zone. Some of us never sleep the night through whether it be because of taking turns on watch or just because of where we are sleeping. And you see, sleeping and particularly REM sleep is the bodies time to process our daily input. So if you can imagine 6 - 12 months in a combat zone, there is a shit load of stuff unprocessed, especially painful memories.
Generally after a month or two of being home we have processed the majority of stuff, but some of it is way too nasty so it stays in the forefront of our mind and is left unprocessed. Therefore we experience flashbacks, and certain things trigger the painful memories and we can have nightmares.
Therapy helps deal with that. Exposure therapy is one name branded around, but there are so many. Basically by talking about it and talking through the incident, we can eventually process it. But one incident could take months of therapy. In the interim though, the therapist can help us deal with the symptoms of PTSD itself.
Now medication. I am not talking about sleeping tablets, all they really do is alter our consciousness, they don't really help with sleep, just like alcohol. The medication that gets prescribed helps your mind relax so the therapist can do their job.
The problem with all of these is that it can sometimes take a while to find the right combination, then again you could be lucky and find the right therapist and get the right medication straight away.
Asking on this forum can help too. Ask any question in the relevant section, i.e. relationships, therapy medication, and we will give our idea's and opinion, it's how I did it.
The biggest thing Rez is that PTSD is hard work. It can get you down too, but you have to want to beat that beast.