Particle physics is the study of fundamental particles and the forces that guide them. Because many of the fundamental particles only appear during relativistic collisions in particle accelerators, colloquially called "atom smashers," particle physics is also known as "high-energy physics". Physicists have been slamming together particles at extreme speeds since 1929.
The best picture of particle physics we have today is called the Standard Model, developed painstakingly in the 1970s. It was a reaction to the "particle zoo", a huge proliferation of unusual fundamental particles discovered during high-energy physics experiments in the 1950s and 1960s. The final particle count ended up to be around 31, including 24 fermions (quarks, electrons, neutrinos, and their antiparticles), 6 bosons (one of which, the graviton, has yet to be observed), and one elusive particle responsible for the property of mass itself, which also has yet to observed, the Higgs boson. Basically, the fermions make up matter and the bosons mediate interactions between matter. The light coming from your computer screen is made up of photons which are bosons. They are interacting with the fermions making up your eyeball.
Most of the matter all around us is composed of only a few fundamental particles: up quarks, down quarks, and electrons. There are also 50 trillion low-mass neutrinos streaming through our bodies every second, passing through the entire Earth almost as if it wasn't even there. Neutrinos, whose name means "little neutral particle," are so elusive that it wasn't even known that they had a mass until 1998. One of the newest areas of astronomy is known as neutrino astronomy, whereby neutrino flux from the Sun and supernovae are observed using huge detectors.
Although only a few of particles in the particle zoo make up the matter we are familiar with, particle physics gives us great insight into the structure of reality by showing us the less common variants and how they fit together into a unified family. Particle physics can be said to be responsible for the existence of nuclear power, nuclear medicine, and nuclear bombs. Particle physics is considered one of the most respected areas of science because it ends up producing insights which are useful to other areas, sort of like math.