Just like, "What is abstract photography?" there are many possible answers...
"All photography is an abstraction of reality. All photography is to some degree an abstraction. Just as all photography is driven, to some degree, by a concept. All photography has an indexical relationship to what it is referring to... all photography has a fictional relationship to it as well. As much as it is indexical it is also fictitious."
Adam Broomberg and Oliver Chanarin in the video, "What is conceptual photography (Part 3):
Part 1 and Part 2 are also interesting: