A neutron is a subatomic particle. It has no charge and its mass is approximately equal to that of a proton (1.67×10−27 kg). The number of neutrons that an atom possesses determines neutron number and thus its isotope. It occupies the nucleus of an atom along with another subatomic particle - the proton, where they majority of the mass of an atom is found.
For more information see wiki page on Neutrons