The aim of nuclear structure studies is to understand 1) the ground-state masses of the nuclei, 2) the excited states of nuclei, and their links to other structural problems (shape, size, etc).
The nucleus is an enormously complicated, strongly interacting assembly of fermions, and the problem of nuclear structure is thus enormous. However, we can consider two main aspects, and cover important aspects of both. The first is the collective behaviour of nucleons. At its simplest, this allows us to gain a rough understanding of nuclear masses (the semiempirical mass formula) by treating the nucleus as a liquid drop; we can gain some understanding of excited states by cosidering rotation and vibration of this drop. The second is the microscopic or single particle behaviour of nuclei. This leads us to consider simple shell models of the nucleus.