In this theory there is no microscopic matter as we know it, nor is there any gravity. Strings are tiny, circular, one dimensional shapes, exactly like elastic bands, but with no width.
All strings are identical.
According to string theory, the different particles are simply the outward manifestations of the many different ways the strings can vibrate. We know that guitar strings have many vibrational modes that produce different sounds. Well, these strings also have different vibrational modes, and each mode looks to us like one of the observed elementary particles. In this model gravity too is accounted for by one of the infinite vibrational modes of the string. In fact, even our notions of space and time will need to be revised.