Physics is a science based in nature. It is the study of matter, and how that matter moves through space time. Force and energy are two important studies in the field of physics. Understanding how the universe acts and behaves is an important piece to physics. Depending on exactly what part of the universe is being studied, and how, depends on the branch of physics that is being used.
Thermodynamics studies the effects of temperature changes, pressure, and how much volume is on physical systems. The amount of energy used, and the heat and pressure that it creates is some of what is studied in thermodynamics.
Physical theory is used by theoretical physics. Theoretical physicists try to understand the world. To do this, they make models of reality, and try to rationalize, predict, and explain physical phenomena throughout the universe.
The study of motion of bodies of all different types and sizes of bodies is mechanics. Mechanics is studied at all different levels from too small to be seen all the way up to a universal level.
Astrophysics is a branch of astronomy. It studies universal physics, and more specifically, the physics of the universe. Galaxies, stars, and more are studied in astrophysics.
Geophysics on the other hand, studies properties and the actual physical characteristics of the earth. meteorology, seismology, and oceanography are all sub categories of geophysics.
Atoms and molecules are studied in atomic and molecular physics. The actual characteristics and structures of these tiny particles are what these scientists are most interested in.
There are also other physics branches that focus on different areas of the universe. Physics is known as one of the oldest forms of science, and is always evolving. Astronomy is touted as the oldest form of physics, and is still practiced today. Between the three main branches of science, physics, chemistry, and biology, we are able to study how and why things work together.