it depends...how is that for an answer Sometimes it is a flatbread pizza, some times a bruschetta like dish, sometimes a flat piece of bread with toppings...you kinda have to read the description and that will tell you what it is. Not sure exactly if there is a way to answer your question without a specific dish from a specific restaurant being described.
Flatbread definitely does vary from place to place inside WDW. I've gotten flatbreads from the carts around Adventureland and it's basically been a chicken salad with a giant, chewy piece of pita-type bread underneath. It wasn't really a hot dish.
At the restaurants, flatbreads will be different because they're cooked. I would suggest looking on flickr for pictures of flatbreads! I just found a ton of stuff on flickr just by searching Disney Flatbread.
Thanks, y'all! Mama Melrose was one of the restaurants in question, actually.
I appreciate the responses. When I look at the descriptions, they LOOK like some type of pizza, but it's so hard to tell when they don't just call it "pizza." LOL!