Kashif's Musical Instrument Blog #1:

Guitar:

As the strings are "plucked" it makes a sound. Also the person has to hold the strings that are located in front of the guitar as he/she plucks the strings at the end. By holding the strings at the front of the guitar, this produces different sound. Since the guitar has a lot of strings, they all have different sound to it. To change a note, the person has to move to the desired string at the front of the guitar and at the end. This change the note and the sound. 

http://www.youtube.com/watch?v=F6gPWc_AWVM