An acoustic guitar is one that relies solely on acoustic methods to project the sound produced by its strings. Interestingly, the term "acoustic guitar" was not coined until the invention of the electric guitar made it necessary to differentiate between the two. Most people are familiar with the appearance of an acoustic guitar. Made of wood, hollow, and usually possessing six strings stretched over a "sound hole" on its face, an acoustic guitar is the image that many people associate with the word "guitar."

Instruments related to what we would call a guitar have been in existence for at least 5,000 years. As best we can tell, the guitar seems to have evolved from an instrument called the cithara (lat.), which was used in ancient central Asia. While it may have been widely used for thousands of years, there is little hard evidence of its use until the Middle Ages. At that time, guitars with three, four, and five strings are known to have existed and been popular.

By the time of the Renaissance, which began in the 14th century, the instrument was firmly established enough to warrant its own name, this being the Renaissance guitar. This guitar, sometimes called a gittern, was carved from a single piece of wood, had four strings, and was noticeably smaller than the guitars of today. After the end of the Renaissance, the guitar continued to be a popular musical instrument. The classic appearance with which we are familiar was created by Antonio Torres Jurado in the middle of the 19th century. However, as evinced by the plethora of instrument styles available today, the guitar continues to evolve according to the popular music of the day

Today, acoustic guitars actually fall into two categories, those with steel strings and those with nylon strings; and within these two categories, there are a variety of subcategories. Those guitars with nylon strings include the Renaissance, Baroque, Romantic, Classical (considered the modern version of the original guitar), and Flamenco guitars. Those with steel strings include the Resonator, Archtop, Battente, Lap steel, Lyre, twelve string, and basic steel-string guitars. Not surprisingly, considering the variety of instruments within each category, acoustic guitars can be used in many different kinds of music, from country, to rock, to jazz. Indeed, both historically and in modern times, acoustic guitars are much more varied in their design and construction than their electric cousins.

No matter the category, all acoustic guitars rely on a soundboard and a resonant cavity to produce a discernable sound. The body of a guitar is hollow. When the strings stretched over the sound hole are caused to vibrate, they pass that vibration on to the soundboard. Because the soundboard possesses a larger surface area, it displaces a larger volume of air. The result is a louder sound than can be produced by the strings alone. This sound is then increased once again by the resonant cavity, or the hollow space within the guitar. At no time is any external energy or equipment used to increase the sound produced by an acoustic guitar, which is, of course, what makes it fundamentally different from an electric guitar.