Physics is the main branch of science. It asks fundamental questions like "How did the Universe begin?" and tries to answer them through observations and experiments. What Physics really tries is to summarize everything happening in a system, every observational phenomenon, such as the Universe itself, through a single, simple, elegant theory, a mathematical model that can later be used to provide outcomes of such phenomenons.
But laying one theory to explain everything in a system is quite difficult, if not impossible, so physics was divided into branches and subbranches. Chemistry was introduced to deal with the forces at molecular level, while Biology was introduced to deal with the happenings in cellular levels. Physics itself was later divided into other simpler branches to make things simpler, like theoretical physics, applied physics, nuclear physics, astrophysics, and so on. Each describes the forces in their respective fields.
The ultimate goal of Physics is to one day sum up the entire Universe, every event in it, every force, every reaction, into one elegant theory, the so called Grand Unified Theory (GUT). Such a theory will combine every branch of science, all of physics, chemistry, biology, economics, and so on, into one. With the help of GUT, one would be able to predict, and prove, the outcome of virtually any event in the universe.
Although there has been significant advancement towards one such theory, we haven't been quite able to get there yet. The main obstruction is that scientists don't really know whether such a theory exists. And hence, the search is still on, with experiments sometimes showing hopes of such a theory at disapproving it at other times.