The Parts of a Guitar

The Headstock

Tuning Pegs
Turn them to tune the guitar. By turning these, the string posts increase or decrease the tension of the strings, allowing them to change pitch. Depending on which way you wound your strings initially, you may have to turn the knobs clockwise or counter-clockwise.
String Posts
Metal pieces around which the strings are wound. Turn the tuning pegs to wind the strings more or less around the string posts. As mentioned above, this increases or decreases the tension of the strings, allowing their pitches to change.

The Neck

The Fingerboard (Fretboard)
This is the term for the front of the neck of a guitar. This is where the frets are located, and it is where a player puts his or her fingers to play different notes. This is where most of the action takes place.
The Nut
This part sits between the head and the neck. It is grooved to hold the individual strings. In music terms, the nut serves as one end of the guitar's scale length — that is, the range of the strings over which notes can be made.
The term "fret" can actually refer to two different, but closely related, parts of a guitar. First, it can refer to the strips of metal placed on the neck at specific intervals to create different pitches. It can also refer to the space on the fingerboard between each strip of metal, as these spaces are where the player puts his or her fingers when playing notes. From here on out, when I talk about placing fingers at specific frets, I am talking about the space between each strip of metal.
Position Markers
The position markers are simply some image, often a dot, that indicates different frets on the fingerboard. They're really just reference points so you don't lose your place. On my guitar, the position markers indicate the third, fifth, seventh, ninth, twelfth, fifteenth and seventeenth frets.

The Body

This is a very important piece of an acoustic guitar. This is the front panel of wood on the body. In acoustic guitars, it amplifies the sound of the strings. When the soundboard receives the vibrations of the strings, the whole thing begins vibrating. The hollow body of the guitar then allows the sound of the vibrations carrying through the soundboard to resonate.
Sound Hole
Often round and centered, the sound hole is the opening in the soundboard out of which the resonating sound in the body escapes, letting us all hear the guitar.
Some acoustic guitars have body style adaptations called cutaways. They allow the player to reach higher frets on the fingerboard without having to strain. Because of this negative space, the body design must compensate in order to maintain the guitar's sound — that's partly why the body of this guitar is fatter than one without a cutaway.
Tuner and Sound Controls
Acoustic-electric guitars are more expensive than their regular brethren, but they have many advantages. One plus is the onboard tuner, which when turned on allows you to simply tune by picking a string and then looking at the display. These guitars also have sound controls with which a player can control bass, middle and treble levels, as well as the volume, when the guitar is plugged into speakers or an amp.
The bridge serves as the anchor for one end of the guitar strings (the other being the string posts on the headstock). It also transmits vibrations from the saddle to the soundboard.
This is a thin, usually plastic, piece embedded in the bridge. Musically speaking, it serves as the other end of the guitar's scale length (the first end being the nut). Vibrations from the strings travel into the saddle. From the saddle, they travel through the bridge and into the soundboard.
Bridge Pins
These are simply little plastic pins that hold the strings down inside the bridge. They are grooved on the inside to accept the strings.