Okay, so one of last years main subjects (as well as this years) is simple harmonic motion, but thats only a piece of how string instruments and music are created. I personally love the example of cellos (for those of you non string instrument fancy people, giant sit-down violin) since their larger it is much easier to visualize. But what happens is we put rosin (solidified tree sap) on our bows, then pull the bow across the string. By pulling the bow across the string, we make the string act in simple harmonic motion from one end (whether it be the the placed finger or all the way back to the top) down to the bridge. The bridge (a little wooden piece connecting the stings to the body) then transfers the vibrations from the strings' SHM into the body of the instrument. Then the vibrations split in two. one part is thrown across the top of the instrument from the base of the bridge,. Other vibrations are sent through the sound post (a small post in the center of the instrument) down to the back of the bod of the instrument. This causes the air within the instrument to pressurize and depressurize in the compressional wave patterns our ears perceive as sound.
So, as a recap, the vibrations go, across the strings, down to the bridge, down the soundpost into the body, pressurize air, then out the F-holes (which IS the technical name for them may i add) To our ears where we hear anything from screeching, to Bach's concertos
The way in which we change the notes is either by making a harmonic by putting fingers lightly on the string to create a node in which the vibrations can travel through but creating a halved period on the wave. OR by putting out fingers down which created an end node that generates a different period of wave, in western music the intervals are logarithmic (1, 10, 100, 1000) per octave but in eastern music the intervals are perfect on 5ths (0,5,10,15) per octave, which makes for completely different sounds of music, although both can come out beautifully even though completely different in sound.