field of Physics

Physics is a diverse area of study and in order to make sense of it scientists have been forced to focus their attention on one or two smaller areas of the discipline. This allows them to become experts in that narrow field, without getting bogged down in the sheer volume of knowledge that exists regarding the natural world.

Below is a list - by no comprehensive - of different disciplines of physics. The list will be updated with new additions and definitions as appropriate.
Acoustics - the study of sound & sound waves
Astronomy - the study of space
Astrophysics - the study of the physical properties of objects in space
Atomic Physics - the study of atoms, specifically the electron properties of the atom
Biophysics - the study of physics in living systems
Chaos - the study of systems with strong sensitivity to initial conditions, so a slight change at the beginning quickly become major changes in the system
Chemical Physics - the study of physics in chemical systems
Computational Physics - the application of numerical methods to solve physical problems for which a quantitative theory already exists
Cosmology - the study of the universe as a whole, including its origins and evolution
Cryophysics / Cryogenics / Low Temperature Physics - the study of physical properties in low temperature situations, far below the freezing point of water
Crystallography - the study of crystals and crystalline structures
Electromagnetism - the study of electrical and magnetic fields, which are two aspects of the same phenomenon
Electronics - the study of the flow of electrons, generally in a circuit
Fluid Dynamics / Fluid Mechanics - the study of the physical properties of "fluids," specifically defined in this case to be liquids and gases
Geophysics - the study of the physical properties of the Earth
High Energy Physics - the study of physics in extremely high energy systems, generally within particle physics
High Pressure Physics - the study of physics in extremely high pressure systems, generally related to fluid dynamics
Laser Physics - the study of the physical properties of lasers
Mathematical Physics - applying mathematically rigorous methods to solving problems within physics
Mechanics - the study of the motion of bodies in a frame of reference
Meteorology / Weather Physics - the physics of the weather
Molecular Physics - the study of physical properties of molecules
Nanotechnology - the science of building circuits and machines from single molecules and atoms
Nuclear Physics - the study of the physical properties of the atomic nucleus
Optics / Light Physics - the study of the physical properties of light
Particle Physics - the study of fundamental particles and the forces of their interaction
Plasma Physics - the study of matter in the plasma phase
Quantum Electrodynamics - the study of how electrons and photons interact at the quantum mechanical level
Quantum Mechanics / Quantum Physics - the study of science where the smallest discrete values, or quanta, of matter and energy become relevant
Quantum Optics - the application of quantum physics to light
Quantum Field Theory - the application of quantum physics to fields, including the fundamental forces of the universe
Quantum Gravity - the application of quantum physics to gravity and unification of gravity with the other fundamental particle interactions
Relativity - the study of systems displaying the properties of Einstein's theory of relativity, which generally involves moving at speeds very close to the speed of light
Statistical Mechanics - the study of large systems by statistically expanding the knowledge of smaller systems
String Theory / Superstring Theory - the study of the theory that all fundamental particles are vibrations of one-dimensional strings of energy, in a higher-dimensional universe
Thermodynamics - the physics of heat
It should become obvious that there is some overlap.