Pre-Trip reports are basically announcing the trip and telling the story leading up to the actual visit. Planning, etc.
Trip Reports ("post-trip") are the story of the actual trip. I don't do PTRs, so I combine the two into a single report.
What do you put in them? Well, whatever you want! You're the one telling the story, so you decide. Do you want to do text-only, or include a ton of pictures? How much detail do you want to use when describing your experience? Do you write, "Then we rode Splash Mountain", or do you write, "we made our way up to Frontierland, hoping to find a short standby line for Splash Mountain. When we arrived, the board showed a 15-minute wait, so we got in the queue, snapping photos on the way"?
My advice, if you're interested in writing one, is to read some of the existing TRs and PTRs to get a general idea of how different people write them. Then, choose for yourself how you want to go about telling your tale. Read the ones with a lot of views and comments, since they're apparently doing something right
Personally, for me, if I see a TR that doesn't have any photos or doesn't give a lot of detail, I don't read it. I like for the TRs I read to flow like a novel. Make me feel like I'm there with you. Others might disagree, but the most popular TRs do seem to follow that formula.
TRs for me, and many others, are a great way to relive the visit and get over that post-Disney depression. Also, it's one of the few really creative outlets I have. So most importantly, have fun with it!