Sound in brief but remarkeable terms is a vibration, that our ears percieve by the sense of hearing. Most commonly vibrations travel to our ears via the air. The ear then converts these sound waves into nerve impulses that are sent to our brains, where the impulses become sound. To say all that in a more technical language: Sound "is an alternation in pressure, particle displacement, or particle velocity propagated in an elastic material" (Olson 1957). Sound is also a series of mechanical compressions and rarefactions or longitudinal waves that successively propagate through media that are at least a little compressible. What causes sound waves is known as "the source of waves". Examples of sounds sources is: A violin string that vibrates upon being bowed or plucked.
The four characteristics of sound are frequency, wavelength, amplitude and velocity.
The frequency of sound is the number of air pressure oscillations per second at a fixed point occupied by a sound wave.
The amplitude is the magnitude of sound pressure change within the wave. Basically this is the maximum amount of pressure at any point in the sound wave. A sound wave is caused literally by increases in pressure at certain points causing a "domino effect" outward, the higher pressure points are the crests in a sound wave, and behind them are low pressure points which tail them. These are known as the troughs on a wavelength graph. Sound's propagation Velocity depends largely on the type, temperature and pressure of the medium through which it propagates. Because air is nearly a perfect gas, the speed of sound does not depend on air pressure.
The frequency range of sound that is audible to humans is approx. between 20 and 20,000 Hz. This range of course varies between individuals, and goes down as are age increases. Sounds will begin to damage our ears at 85 dBSPL and sounds above approximately 130 dBSPL will cause pain, as a result are known as the: "threshold of pain". Of course again this range will vary among individuals and will change with age.