Nuclear forces are the forces between two or more nucleons. It is the force which binds protons and neutrons into the atomic nuclei. Due to the energy released by the binding of the proton and neutrons, the mass of nuclei will be less than the total mass of the protons and neutrons combined which form them. This release of energy is used in nuclear power and nuclear weapons.
Forces between nucleons at a distance of 1 femtometer between the centers are very attractive; however the attractiveness decreases from distances beyond 2.5 femtometers. When the centers of the nucleons are 0.7 femtometers or less apart the force is repulsive. This is responsible for the physical size of nuclei since the nucleons can come no closer than the force allows.
The nuclear force is a residual effect of the strong force or strong interactions. It is the attractive force that binds quarks together to form the nucleons. The strong force is only sensed from composite particle made up of quarks to form the nucleons which is called a hadron. This powerful force is mediated by a particle called a gluon, which is a type of gauge boson. Gluons hold quarks together with a force like that of an electric charge, but a far greater power.