Scholars use the term "classical physics" to describe theories of physics developed prior to 1900 and "modern physics" as a label for developments that occurred after 1900; classical physics deals with matter and energy on a macro scale without delving into the more complex studies of quanta that characterize modern physics. Max Planck's work marked the end of classical physics. Unlike classical physics, modern physics includes the theory of relativity.
Classical physics was an outgrowth of "natural science," a broad term that initially encompassed all scientific inquiry. As time passed, scientific subfields, like biology and astronomy, began developing. The questions that concern physics--the flow of matter and energy--had always been at the heart of natural science. Classical physics primarily involved mechanics (the study of the forces that affect the motion of objects), hydrodynamics, optics, thermodynamics and acoustics.
Some of the important laws of classical physics are the laws of conservation of mass and of energy. According to these, energy in a system is neither created nor destroyed.
In the twentieth century, Max Planck put an end to Newtonian classical physics with the development of quantam mechanics. His theories regarding quanta, or small units of energy, revolutionized the science. As a result of Planck's work and that of successors like Einstein, modern physics became focused on the structure of nature's smallest particles, rather than on the large, observable systems that are the focus of classical physics.