At its root, philosophy is a rational method, a way of thinking clearly, not a "subject" in the ordinary sense. That is, philosophy does not seek a particular way of understanding human behavior, as does, for example, history or sociology or psychology. Neither does philosophy teach a particular set of techniques meant to help solve empirical problems, as in a science classroom.
Rather, the philosopher is most concerned with conceptual questions, with tackling and (hopefully) "dissolving" conceptual problems. Which conceptual problems exactly? Well, it depends on the interests of the philosopher. Philosophical reasoning improves almost any inquiry, and, as a matter of fact, philosophers might inquire about law, science, art, religion, ethics, and the nature of existence and experience. (Just take a look at our courses offered.) With regard to the content of its inquiries, philosophy can be an exceedingly broad discipline.