Kind of like relativity. However, quantum mechanics seems terribly wrong and very messy. No one knows why quantum mechanics is the way it is but experiments seem to show it is correct.
All strings are identical.
However, what makes the particles different that are made from these strings is the vibrational characteristics of the string. For example, the single string that makes up an electron may vibrate faster and with a larger amplitude than the string that makes up a gluon. These differences in vibration make the mass, charge, etc different for these particles. A faster vibrational frequency with a larger amplitude obviously equivilates to higher energy. So according to string theory the particles have different mass because the strings have different oscillation frequencies and amplitudes thus having different energies and thus different masses.