Abstract art uses a visual language of form, color and line to create a composition which may exist with a degree of independence from visual references in the world. It focuses more on the materials and form of art rather than on the rendering of a recognizable object or the illusion of reality.
Abstract art can be a very formal exploration of the elements and principles of design (form, shape, color, value, balance, harmony, variety, movement, rhythm, line, texture, and composition), or it can be a way to visually represent non-visual things, such as emotion, experiences, and music. It can also refer to work which “abstracts” or simplifies the form of things from the world.
Art that completely eliminates any representation from reality could better be called “non-objective.”
For more detailed information, please see this Wikipedia article.