Physicists are scientists trained in the area of physics, which is the study of the principles of the relationships between matter, space, energy and time in our universe. They may specialise in an academic research area, such as nuclear physics, or may be employed commercially.
Because physics applies to everything in the universe and nobody can ever hope to understand all of it, physicists normally specialise in a subfield.
Here are some examples of popular specialisations:
Atomic, molecular, and optical physicists study the behavior of electrons, light, atoms, and simple molecules. Each of these areas are highly specialised, but there is unavoidable overlap between the specialties.
Astrophysicists study the physical properties of the universe. Their work largely overlaps with the work of astronomers, so astrophysics is a subfield of both physics and astronomy.
Particle and nuclear physicists study electrons, nuclei, quarks, and other subatomic and atomic particles. These physicists often work with nuclear power generation, nuclear medicine, and magnetic resonance imaging.
Medical physicists apply their knowledge of physics to the creation of new medical technologies and treatments. Without medical physicists, we wouldn't have ultrasounds, MRIs, or radiation-based cancer treatments.
Condensed matter physicists study phenomena of condensed phases of matter. If you're not a physicist, you are probably most familiar with liquid and solid phases, but the superconducting phase, ferromagnetic phase, and antiferromagnetic phase are examples of other condensed phases that are often studied. Presently, most physicists are condensed matter physicists.