Nuclear data groups all experimental data relevant for nuclear physics and nuclear applications. It includes a large and non-exhaustic number of physical quantities, like scattering and reaction cross sections (which can also be measured as functions of e.g. the energy and the angle), nuclear structure and nuclear decay parameters, etc. It can involve neutrons, protons, deuterons, alpha particles, and virtually all nuclear isotopes which can be handled in a laboratory.
There are two major reasons to need high-quality nuclear data: theoretical model development of nuclear physics, and applications involving radiation and nuclear power. There is often an interplay between these two aspects, since applications often motivate research in particular theoretical fields, and theory can be used to predict quantities or phenomena which can lead to new or improved technological concepts.