Strong, weak and Electromagnetic forces
Nucleons interact through three different means. These are refered to a "forces" even though "interactions" might be a more appropriate term.
Strong force: This is a short-range attractive force. "Short range" refers to the fact that the force dies off exponentially in distance. This means that a nucleon is only affected by the strong force of its nearest neighbors. However, within that distance of less than 2.0 E-15 m, it overwhelms the other forces.
Weak force: We will not
concern ourselves much with the weak force. It is much weaker and much shorter range than
the strong force. It is principally of interest because it includes interactions which can
change neutrons to protons. 
(a neutron
decays to a proton, an electron and an anti-neutrino). This process is known as beta
decay. Since nuclei prefer to have roughly equal numbers of neutrons and protons, beta
decay provides the avenue for nuclei to achieve the optimal balance of nuetrons and
protons.
Electromagnetic force: The Coulomb force, which acts between protons, is a long-range force. For nucleons which are near one another in the nucleus, the coulomb interaction is dwarfed by the strong force. But in large nuclei, two protons at opposite ends of a nucleus continue to feel the repulsive effects of the coulomb interaction. The coulomb repulsion is responsible for fission of large nuclei. If it were not for the electromagnetic force, nuclei could grow arbitrarily large.