Sound waves are longitudinal waves which means they vibrate in the same direction as the direction of travel. They are produced by vibrating sources, such as speakers. Sound waves can only travel through a solid, liquid or gas medium. They travel fastest in solids, then liquids and slowest in gases. A good model for sound waves is a spring.
The sections of the sound waves where the particles are pushed together are areas of compression. The sections of the sound waves where the particles are further apart are areas of rarefaction.