Abstract photography is not very easy to define - perhaps that should be part of its definition! Nevertheless it is a popular art form where the normal established guidelines for composition, focus, depth of field etc no longer apply. the photographer is limited only by his or her imagination and can use any technique available in whatever way they wish. The only requirement for success is that the image is pleasing to the eye.
Lots of abstract images are created from our everyday surroundings. A perceptive eye will spot potential subjects in all sorts of common objects. Images may be created using combinations of colours, textures, unusual arrangements of light and shade, patterns, or parts of ordinary objects seen in close-up and perhaps at some unusual angle of view. In many cases the viewer will not be able to identify the exact identity of the subject, but that is all part of the fascination of abstract images.
Once created, abstract images may mean different things to different people. Some may see a pattern, others an interesting play of light. Some may see an unique combination of colours while others may see the same image as a section of some object captured from a viewpoint that is not normally experienced.
In the majority of images the viewfinder is filled with the subject so that the viewer sees no points of reference and gets little or no idea of scale. This all adds to a feeling of the subject being unknown or mysterious, and leaves the eye to appreciate the disembodied colours, curves or whatever detail the image offers.
Examples of good subjects for abstract work include reflections in water, running water or other liquids, floral close-ups, large numbers of small but identical objects closely packed to form a pattern, swirls of colour in paint etc etc.