The principle of how an acoustic instrument, in our case the guitar, produces a sound, is the same for all types of stinged acoustic instrument. A string of the guitar thats struck by the pick or finger, vibrates. This vibration is not sufficient to propogate soundwaves that can be heard by our ears. For example, try to play a string on a non-amplified electric guitar: as you will see, the volume of the played note is relatively low and almost inaudible.
For this reason, the acoustic guitar has an empty body, called the resonance chamber. The purpose of this is to amplify the vibration of the string, also corolating with the vibration of the wooden parts: the better the materials used to make the guitar, the better the sound will be. Also, the type of wood used for the construction of the body could change the tone of the instrument itself.
The fundamental difference betwee the acoustic and electric guitars is therefore in the fact that the acoustic guitar naturally amplifies the sound of the string, while the electric guitar uses electronic components.
Up to now we have talked about the acoustic guitar. The term is generic because there are two main catagories of this type of instrument: the classical guitar and the folk guitar. We will now look at the different characteristics of these two types of acoustic guitars.

Takamine EC132C

The biggest difference between the classical guitar (left) and the folk guitar (right) is surely the sound. The classical guitar normally has nylon stings which give the instrument a soft and sweet sound, while the stings of the folk guitar are metalic, giving it a characteristic shrill sound. Other differences between the two instruments is the size of the body, where the classical guitar is smaller. Furthermore, the fretboard of the classical guitar attaches to the body at the 12th fret (the 14th fret for the folk). The mounting of the strings to the bridge and to the headstock are also different for the two instruments. What is substantially similar in the two acoustic instruments is their structure.  Below you can observe the main parts the make up the classical guitar.

Takamine EG334BC

The fretboard (1) is the section of the neck where the notes are fingered. It's divided into various sections, called frets (2), each of which has position markers (3). At the top of the neck is the headstock (4) which has the tuners, or tuning pegs (5). These are used to tune the instrument.

The body of the instrument is formed by two overlapping panels united at the lateral band (6). To the top panel (7) is attached the bridge (8), where the strings are anchored. The soundhole (9) permits the exit of the sound waves created by the vibration of the strings.

No precise documents exist regarding the first development of the classical guitar. Some information from guitar texts indicate in the 12th century the first clear signals of an instrument with characteristics similar to todays classical guitar. But its very probable that the ancestors of the instrument were ethnic instruments, like the lute, the rebec and various string instruments in use in medieval Europe. Spain certainly was a excellant "breeding ground" for the classical guitar. Just think of a past genious like José Ferdinand Sor (1778-1839). Every classical guitarist, even today, has in his repertoire one or more of his songs (there are more than 400). The most recent of the great Spaniards was surely the Master Andrès Segovia, but in these centuries also Europe has seen the evolution of great guitarists (to name only a few: Carcassi, Giuliani, Aguado, Tarrega) as well as the evolution of the instrument to the actual form that we know today.
The main characteristic of the sound of the classical guitar is in the string, which are nylon (the first three) and metalic material (the other three). As already stated, the resulting sound is very soft and sweet, thanks to the material of the strings.
The flamenco guitar is a cousin of the classical guitar and is used to play this type of music. The main difference is the resonance chamber, which is slightly smaller with respect to the classical guitar. Furthermore, there is a special protective cover mounted on the top panel of the chamber, used by flamenco guitarists to obtain the "golpe" technique with the fingers of the right hand, frequently used in this type of music.

Headstock

In the classical guitar, the strings are anchored to the head on open type tuners, mounted on the sides of the head. The strings are connected to the bridge through specific holes.

Bridge

We previously saw the main differences between the classical and the folk guitars. We need to add the fact that the sound from the strings, besides being more shrill, is also louder with respect to the classical guitar. This is because the body of the folk guitar is much bigger and therefore gives a greater response with respect to the movement of air.

The strings of the folk guitar are anchored to the head on closed type tuners, mounted on holes that pass through the head. On the bridge, the strings are fixed within special slots and are blocked by pegs that are inserted by pressure.
In the nex page we will analyze the various parts that make up the acoustic instruments, both classical and folk.