The Strong Force (or Strong Interaction) is one of the four known fundamental forces (along with Electromagnetism, the weak force, and Gravity). Unlike the electric force or gravity, it is not described by an Inverse Square Law, but is extremely strong (enough to overcome the electrical repulsion of protons and charged quarks) at very short distances, but falls off abruptly by distance. At the small distances in question, the ratio of electric to strong forces is approximately 1/137, the number being the Fine Structure Constant. The strong force is the force that must be overcome for Fission to occur.
Electroweak theory treats electromagnetism and the weak force as two aspects of a single force. Proposed theories positing a single force that also includes the strong force as a third aspect are termed Grand Unified Theories or GUTs.