There is no single unified theory or practice of astrology. Ancient cultures all practiced their own forms, some of which combined and evolved into today's common western astrology. Eastern cultures continue to practice their own forms of astrology:
Chinese, Vedic and Tibetan astrology are among the most well-known.
Even within western astrology, there is a considerable diversity of methods and philosophies. Some divide astrology by the end result that is intended:
Mundane Astrology - This is used to examine world events and make predictions about national affairs, wars and economies.
Interrogatory Astrology - This branch can be further subdivided, but generally refers to astrology that seeks to make specific predictions or analyses about the subject's objectives or events within the subject's life.
Natal Astrology - This is what most people think of when they think of astrology. Natal Astrology seeks to make predictions and analyses based on the date of a person's birth. It's based on the idea that everything that happens to something is expressed at the very beginning of that thing, sometimes known as the Law of Beginnings