Violin is one of the string instruments family
 
String instruments, just as their name implies, make the use of a vibrating string or cord to produce sound. Instruments in this family include pianos, violins, violas, cellos, guitars, banjos, etc. While the method of how the string is set into vibration varies in string instruments, the basic principal behind them is the same: Different strings vibrate at different rates thus producing different sounds. And by adjusting the tension, thickness, and material of the string, as well as how the string is set into vibration, different tones and frequencies are produced.
 
In general, the thicker and heavier a string is, the lower of a sound is produced. And conversely, the thiner and lighter a string is, the higher the sound produced is. Along with the weight of the string, the legnth of the string has a great bearing on the sound that it produces. In instruments where the string legnths vary, such as harps or pianos, the thicker strings are also longer and produce a lower sound. However, tension also has a lot to do with what frequency a string will produce. A more tightly strung thick string may very well produce a higher note than a losely strung, thin string. In fact, by adjusting the tension of the strings on any given instrument, the instrument can be tuned. (To tune an instrument is to calibrate it to produce the correct sounds.) The other main factor that affects the tones of a string instrument is the material that the string is made of. While in older days, strings for such instruments may have been made of many different materials, today, most strings are made of metal cord or cable. However, the metal used does vary to some degree. The metal, of course, comes in many different thicknesses. Many metal cords are even coiled much like a coiled telephone cord (but much tighter, of course). The cords also vary in the type of metal that is used to make them. Steel is the predominate metal used for the cords.
 
Piano and guitar strings are very similar. If this is the case, then why do the two instruments sound so different? The answer is acoustics. Acoustics, in this sense, is the way in which the sound from the string is affected by the instrument. For instance, the piano makes use of a hammer hitting a string while a guitar is plucked. Also, the piano's sound resonates off it's inner chamber while a guitar's sound resonates through the large hole under the strings. (In both cases, the electronic versions use a pickup to turn the sound into an electronic signal that can be passed to an amplifier.)
 
String instruments can be very different in size, shape, and the sounds they produce, but they all use strings to make their sounds. As mentioned above, the sound produced is a result of various lengths, materials, types, and tensions of the strings as well as the type of instrument that the sound resonates from.