Waves on strings are generally transverse which can then set up longitudinal waves in the surrounding air.  Standing waves are created by reflections from the nodes at either end of the string.

The simplest wave that can be set up on a string is called the fundamental or first harmonic and all other waves are known as overtones.

When a wave travels from one medium to another, its speed changes.   This means that there is also a change in direction when the wave is incident at an angle.

The frequency of the wave produced on the string is dependant only on the string's length (l), its mass per unit length () and tension (T).

When two waves meet, interference occurs.  This can be either constructive or destructive.  Their amplitudes are added or subtracted with respect to each other to create a new and different sounding wave.  When interference takes place, energy is not destroyed or created, merely redistributed.