Nuclear physics is the study of the atomic nucleus.
Classical nuclear physics has considered the nucleus as consisting solely of protons and neutrons held together by the strong nuclear force. It is concerned with the structure and reactions of nuclei, as they relate to and reveal the nature of the forces between the nucleons. It also encompasses such things as the origin and nucleosynthesis of the elements in astrophysical processes.
These days the boundaries of nuclear physics are more complicated, and include such topics as the relation of the nuclear force to the underlying strong forces between quarks and the behaviour of nuclei under extreme conditions, for instance the possible formation of a quark-gluon plasma in energetic collisions.
It might be thought that we should really be studying the physics of large collections of quarks. To date, however, evidence of the quark substructure of nucleons is extremely difficult to find in nuclear physics.
The basic properties of nuclei indicate that the nucleus is held together by short range forces. This means that experiments that explore the force between two nuclei must be microscopic, i.e. the nuclei must be brought into close proximity, of the order of the range of the force (typically 1 fm) in order for effects to be observed. The strong electric repulsion between the nuclei paradoxically implies that these experiments require large accelerators and high energy particles.