Body

Picture from: http://www.musicdistributorsworldwide.com/Davisonguitarpics/jameson%205%20string%20acoustic%20bass.jpg

    The body of the guitar serves to transmit the vibrations of the strings to the vibration of the air. The top plate of the guitar is made to vibrate up and down easily, usually made of spruce or other springy wood and about 2.5 mm thick. There are series of bracers on the top plate to strengthen it and keep it flat. These bracers also affect the way that the top plate vibrates. The back plate does not play as much of a role in the vibration, in part because it is held against the player’s body. The sides of the guitar do not vibrate much perpendicular to its surface so it does not produce much sound. The interaction between the bridge and the strings is very important to transfer the sound to the air. The string frequency vibrates the bridge with vibrates the top plate and causes the air to vibrate.
 Here is a picture showing the bracers on the inside of the guitar body: