The word "physics" comes from the Greek "physis," meaning "nature." In it's broadest sense, physics is the study of nature with the objective of understanding how the universe behaves. It is often considered the "fundamental science" because it deals with the most basic building-blocks of our universe. New ideas and discoveries in physics are often highly influential in other fields, and many scientific disciplines arose originally from research conducted in physics. Fields as diverse as chemistry, quantum mechanics, electronics, and astronomy can all be viewed as applications of physics. It is a very broad science which covers everything from the behavior of particles on a sub-atomic level to the motion of entire galaxies through space, but physicists are primarily concerned with matter, energy, and the interactions between them.
Because it covers so much ground, modern physics is divided into numerous specific fields of study. Below is an overview of some of the major fields in contemporary physics:
Particle physics, also known as high-energy physics, is the study of the most elementary building blocks of matter and radiation. Particle physicists use high-energy particle accelerators to collide sub-atomic particles with each other and study their behaviors.
Astrophysics deals with the physics of the universe on large scale. Astrophysicists study the physical properties of objects in space and their interactions with one another.
Atomic and molecular physics are branches of physics which study the interactions between single atoms or structures containing only a few atoms as well as their interactions with light.
Acoustics is the study of sound. Acousticians study the physical properties of sound waves and their interactions with other forces and their environment. The application of acoustics in technology is known as acoustical engineering. Acoustical engineers may be involved in the design of concert halls, recording studios, or loudspeakers for optimum transmission of sound.
Condensed-matter physics is the study of the physical properties of matter in condensed states. The condensed states of matter include solid and liquid as well as less common phases such as superfluids. Condensed-matter physics is the largest field in contemporary physics.
These are just a small sampling; there are many more sub-fields within physics, many of which may be divided even further. The boundaries between them are often blurry, and the work of a single physicist may cover multiple different specific fields of study.