Branches. Founders. Modern physics. Classical physics is usually concerned with everyday conditions: speeds much lower than the speed of light, and sizes much greater than that of atoms.
Modern physics is usually concerned with high velocities and small distances. The term modern physics refers to the post-Newtonian conception of physics. Put simply, modern physics deals with the underlying structure of the smallest particles in nature ("quantum" mechanics), as well as a rigorous understanding of the fundamental interaction of particles, understood as forces. Small velocities and large distances is usually the realm of classical physics.