An acoustic guitar is a guitar that uses only acoustic methods to project the sound produced by its strings, as opposed to one that requires an amplifier.
The 6-string guitar is the modern descendant of a sub-family of the "chordophone" class of vibrating string instruments, from which the bowed string family such as violins, and fixed-length plucked string instruments such as the harp and lyre, are also derived. The name comes from the Latin "cithara" and related Greek "kithara", which themselves are the name of an ancient instrument more like a lyre or harp. The first examples of instruments resembling the modern guitar are from the Andalusian region, about the 12th century, believed to be derived from the European lute and related Moorish oud. These evolved into the Spanish "vihuela" or "viola de mano", a fretted instrument with a "waisted" body closely resembling the Baroque guitar, which is the direct ancestor of the modern classical and steel-strung acoustic guitars.
Modern acoustic guitars come in two basic types; the nylon-strung, cross-braced "classical guitar", used today mainly for various chamber music and some Latin styles, and the newer steel-string guitar, first appearing in the mid-1800s with the development of modern "X-bracing" techniques by German-American luthiers, including the well-known C.F. Martin. Steel-string acoustic guitars are used for a wide variety of popular music genres beginning around the American Reconstruction and continuing to the present day.
The steel-strung guitar is available in a variety of body styles, each offering a different tone to the guitar based on the relative difference in space and shape between its "upper bout" (the area between the soundhole and the base of the neck) and "lower bout" (the space between the soundhole and bottom of the guitar, incorporating the bridge). The "Double-Oh" or "Grand Concert" design most closely resembles the nylon-strung classical guitar in appearance and sound, with a balanced but somewhat quieter sound than other styles, and is prized by fingerstyle players for its clarity and small, comfortable body shape. The larger "Triple-Oh" or "Grand Auditorium" style increases the size of the lower bout, increasing the presence of bass frequencies and also increasing the overall volume; this is a popular style among all types of players for its more well-rounded tone as heard by modern listeners accustomed to added bass, and is a favorite style among players of the Taylor brand of guitars. The "Dreadnought" style was an attempt by C.F. Martin to further increase the presence of bass tones in a relatively compact body shape, by drastically reducing the size of the upper bout in favor of the lower bout. The resulting roughly wedge-shaped design has become the most popular overall body shape among modern players, and catapulted the Martin name into prominence. Meanwhile, Gibson Guitars introduced the competing "Jumbo" body style, of which the J-200 is a classic example; the guitar's overall body size is much larger, with very pronounced "waisting" between upper and lower bouts, and a large, round lower bout emphasizing bass frequencies. This style is also a favorite among contemporary guitarists, especially in country, though its large size can make it more difficult for smaller-framed players to hold comfortably.
The steel-strung acoustic guitar can be heard in virtually every major genre of contemporary music, being most popular in country-western music and in certain styles of rock music. While it's utilized by bands across the rock spectrum, its use by bands notable for "heavier" rock styles is typically either in "downtempo" songs and styles, such as ballads, or in "unplugged" covers of songs originally recorded using highly-distorted electric guitars. Regular use of acoustic guitar instead of or supplementing electric is more common in "cleaner" rock styles such as folk rock, soft rock, rockabilly and other "adult contemporary" sub-genres.