According to string theory, the most basic indivisible objects that all matter is made of are strings. Strings are vibrating, one-dimensional loops or segments of energy. Unlike violin strings, these strings are not made of atoms which are made up of electrons, protons and neutrons or even quarks. In fact the atoms and all of the particles in them are made of strings. String theory states that each particle arises from the vibrations of fundamental underlying strings, and it is the character of that vibration that determines a particle's properties, such as mass and charge.
Source: New Scientist 17/9/05