I am a college freshmen who is interested in studying Physics, but I do not understand how someone specializes after or during graduate school. I am assuming that your specialty is just what courses you end up taking in graduate school, what field you write your thesis in, and/or the type of research being done at your university. Is this right? Is it possible to specialize in one field, but also have some type of sub-specialty in another? I was also wondering what type of specialties there are (I only know of Astrophysics, Condensed Matter, String Theory(Don't know if this counts), Optics, and Cosmology)? Any input would be great, thanks.