The acoustic and electric guitars have three parts, the neck, head,and body. The body is the part that amplifies the sound but in very different ways.

For the acoustic guitar, the most important piece of the body is the soundboard. This is the wooden piece mounted on the front of the guitar's body, and its job is to make the guitar's sound loud enough for us to hear. In the soundboard is a large hole called the sound hole. The hole is normally round and centered, but F-shaped pairs of holes, as in a violin, are sometimes seen. Attached to the soundboard is a piece called the bridge, which acts as the anchor for one end of the six strings. The bridge has a thin, hard piece embedded in it called the saddle, which is the part that the strings rest against. When the strings vibrate, the vibrations travel through the saddle to the bridge to the soundboard. The entire soundboard is now vibrating. The body of the guitar forms a hollow sound box that amplifies the vibrations of the soundboard. If you touch a tuning fork to the bridge of a guitar you can prove that the vibrations of the soundboard are what produce the sound in an
acoustic guitar.

Most popular electric guitars have solid bodies. The sound is produced by magnetic pickups and controlled by several knobs. If you pluck a string on an electric guitar that is not plugged in, the sound is barely audible. Without a soundboard and a hollow body, there is nothing to amplify the string's vibrations. To produce sound, an electric guitar senses the vibrations of the strings electronically and routes an electronic signal to an amplifier and speaker. The sensing occurs in a magnetic pickup mounted under the strings on the guitar's body. A simple magnetic pickup looks like the picture on this link

This pickup consists of a bar magnet wrapped with as many as 7,000 turns of fine wire. The coiled wire and magnet turns the motion into electrical energy. In the case of an electric guitar, the vibrating steel strings produce a corresponding vibration in the magnet's magnetic field and therefore a vibrating current in the coil. The pickup's coil sends its signals through a very simple circuit on the guitar to a jack. From the jack, the signal runs to an amplifier, which drives a speaker.