There are four known fundamental forces in nature that govern the types of matter that we know and their interactions. These forces are depicted on the clover leaf below. Click on a leaf and you will recover the Wikipedia definition of the force. All the forces are believed to be different phases (steam and ice are just different phases of water) of a single unifying force. The mathematical description of this unifying force is an active area of theoretical particle physics. String theory is an example theory of such a theory. Click on the center of the clover to learn more about the unified forces from Wikipedia.