In quantum field theory all elementary particles are supposed to be point-shaped, it is what we call a theory of point-particles. In string theory the elementary contituents are one-dimensional extended objects, so called strings. The basic idea is that all matter is formed from one type of string and that different elementary particles are just different excitations of the same string. These strings should be very small, so that at ``low'' energies (equivalent to length scales downto cm) all particles look like points and can be adequately descibed by ordinary point particles theories. Only when we magnify to scales of the order of the Planck-length, cm (or equivalently GeV), the extendedness of string becomes apparent.
One of the major motivations for studying these string theories, is that one of the oscillations of the string is a massless, spin 2 state that can be identified as the graviton. String theory thus necessarily contains quantum gravity and is therefore a promissing candidate for a Unified Theory of all four interactions.