Wikipedia refers to abstract art as "art that does not depict objects in the natural world, but instead uses color and form in a non-representational way."
Some might feel that the concept of an "abstract" photograph is a contradiction in terms, since it does, in fact, represent the "real world." I disagree: A photograph that has been captured from an existing form in reality does not necessarily make it any less an abstraction. For instance, If an image captures only a portion of a real form until it no longer resembles that form, it has been abstracted from reality. If it distorts or minimizes an actual form, it has also become a separate and distinct piece of art.
Abstraction in photography should be viewed in the same way as it is in art. One might compare it to Picasso's abstractions of a guitar, which were rendered using an actual guitar as his model. Because a photograph is captured with a camera, it is still done so from the perspective of a photographer with a particular point of view. And from my point of view, an abstract painting and an abstract photograph are both art.