Physics is the study of matter and energy. Since matter and energy are the two things that the entire universe is made of, physics is really the foundation of all the sciences.
For convenience, most people divide physics up into more manageable fields. Most high school physics classes concentrate on the field known as classical mechanics, which is the study of how forces act on objects. In this part of physics, one would study Newton's Laws of Motion, how gravity and friction affect motion, and so on. One would also learn how to use mathematical equations to describe or predict the motions of bodies.
It turns out that classical mechanics doesn't apply 100% of the time. In modern physics, one would study the extremes of the universe, examining the scenarios where classical physics breaks down. Modern physics studies the extremely small (subatomic particles), the extremely large (galaxies), the extremely fast (the speed of light).
Physics is also divided into experimental versus theoretical physics. Experimental physics is the main tool of mechanics, while theoretical physics works on matters that we are not able to manipulate directly and can only model, such as the Big Bang.