Nuclei contain both protons and neutrons. They have a proton number Z and a neutron number N that represent the number of protons and neutrons within the nucleus respectively. These protons and neutrons are held together by the strong nuclear force, while the protons repel eachother via the Coulomb interaction.
Nuclei also gain properties from their shape (prolate / oblate / …) and their spin I. There is not yet a single model that describes the properties of all observed nuclei.
They can be stable or unstable, in the case of an unstable nucleus, the nucleus will emit or capture a proton or neutron, or even turn neutrons into protons and vice versa.