I am currently working on an advanced Audacity tutorial. I begin with sound itself, what is is, how is measured and illustrated visually, etc. This blurb comes from the beginning of my tutorial.
Sound is a disturbance of our surrounding atmosphere that we humans can hear. The air is composed of molecules. (artsites.ucsc.edu). Air vibrations can be easily illustrated using the tuning fork as an example. When you strike a tuning fork against a hard surface its two prongs start to vibrate and the air molecules around them become more densely packed and increase the air pressure slightly. This process is called compression. After a few seconds the molecules spread out again and the air pressure drops. This process is called rarefaction.
These outward vibrations in the air pressure surrounding the turning fork (or any other vibrating object) are called sound waves. Sound waves do not move up and down in the manner of ocean waves. They move longitudinally in a line between the listener and the sound source. In digital audio software air pressure fluctuations may be visually illustrated in a graph such as the one below. In digital audio a graph such as this is called a waveform.