It's fine art if the photographer decides the picture belongs in a gallery or museum (while it can be sold it has to fit the artist's vision, and it's commercial value is less important).
It's commercial art if the photographer tries to sell prints to hang on the walls of homes and businesses (so it must be an image that can be sold, even if it doesn't completely fit into the photographer's vision).
It's photojournalism if the photographer tries to capture something that has general interest exactly as it is in real life (so it has to tell the "truth", commercial value is only a secondary consideration and the artist's vision is irrelevant).
Etc. etc. etc.
"fine art" doesn't have to be any good and it doesn't has to be accepted by experts of other artists (or any guild), there are a lot of artists creating bad art other people don't appreciate - it doesn't make it any less artsy.
Update: didn't mean to say fine art can't be sold, only that the artist is more concerned that the picture fits some vision/theme than with it's commercial value.