Before everything from books to porn or, in this case, guitars were electronic, there existed a thing called the Acoustic Guitar. Anthropologists believe these were once used to create music, but are now mostly used as talent beards.
What you think probably think of when you hear the words "acoustic guitar" is an instrument derived from the six-stringed spanish (or classical) guitar. This was pioneered by Ferdinando Gagliano in the late 18th century and Antonio Torres Jurado in the 19th century. While the classical, six-stringed guitar is now the standard for western music, there are a number of variations across the world. The Russian guitar, for example, has seven strings tuned to an open G chord, while the Baroque guitar has nine to ten strings, consisting of five strings and five courses (smaller strings tuned an octave higher). A twelve-string guitar is a six-string guitar with six courses, and so on. There are as many variations on the acoustic guitar as there are burn-outs with time on their hands.
Here's a hint: it's a lot.
Many different genres of guitar music have been created, often as a result of the poorer and less educated classes gaining access to it. When poor Europeans began to play guitar, they made folk music. When poor black southerners began to play guitar, they created the blues. When poor hippies got the guitar, they created jam bands. When future cultures look back at what they can scavenge of our artistic history (a difficult task, what with all the rampaging nuclear mutants roaming our devastated libraries) they should note that nearly all musical innovation since the classical era has come from the poor.
Today, when most Americans think of acoustic guitar, they think of this:
And promptly start beating the shit out of their ears for facilitating this horrid abuse of sound. However, they can also be used for this:
Which is far too insane and confusing an orgy of sound to be actually considered music, but very well may be the Chord Apocalypse.
Of course, some may insist that people create music for reasons other than to render themselves more attractive to the opposite sex. They say that some people play purely for the joy of creation, and in fact have no further aspirations towards mating. They say many of these people, throughout the ages, have actually chosen isolate themselves and even intentionally remained celibate - completely alone and unacknowledged - spending their lives toiling away in obscurity simply for the love of music and music alone. And of course we know these people exist:
They're called drummers.