What creates sound?
As you can probably guess, vibrations create sound waves. You can feel your speaker vibrates when you turn your music up really loud, and you can see guitar strings vibrate as they create sound. The vibration of an object, whether it be the speaker's system or the guitar strings, causes the air surrounding it to vibrate too. That vibration of air continues to spread out and eventually causes a very small vibration in your eardrum, and that is what your brain interprets as "sound".
Retrieved from http://www.physicsclassroom.com/mmedia/waves/gsl.cfm
Retrieved from http://newt.phys.unsw.edu.au/music/guitar/guitarintro.html#First
Near the object making sound are molecules that are vibrating very quickly. Not only that, but the air pressure also goes up and down, although it is a very small amount. These total vibrations are measured over an individual second, that is called the frequency. Frequency is measured in cycles per second, called Hertz (Hz). The pitches for any note is determined by the frequency, which is directly proportional. High frequency means a higher pitch and vice versa. For example, if we take a guitar and and pluck the A string, we'll get 110 vibrations over one second so that makes it 110 Hz. Plucking higher notes gives us a larger amount of vibrations per second.