Sound travels through a medium as waves.  These waves are created by the vibrations.  So if you jiggle your keys or strum a guitar the motion creates waves which travel to the ear drum, a receiver of sound waves. 

Sound will travel through any medium that is elastic.  It will not travel through the vacuum of space. 

Sound has a characteristic waveform and is harmonic.  These waves have amplitude which is associated with the intensity of the sound.  A loud sound has a higher amplitude than a soft one.  Also the intensity falls off the further away it is from the source of the sound itself. 

Sound is also a compression wave.  It alternately causes molecules to compress together and then to pull apart which is known as rarefaction.  

Sound has pitch also.  The more wavelengths per unit time the higher the pitch.  Sound can have so high a pitch that humans can no longer hear it.  (Dogs can hear some wavelengths to short for humans.)  This sound is called ultrasound.  The really long waves are not detected by the human ear either.  These longer waves are known as infrasound.

For more information see:

http://www.school-for-champions.com/science/sound.htm