Six strings of pure awesome.
The guitar is a stringed instrument that has its roots in the Roman cithara, the Scandinavian lut, and the Spanish vihuela, though all of these were in fact pretty lame and had not even the slightest inclination of Rock in them.
Not Pictured: Wicked solos.
The modern, 6-stringed guitar came into being around the 16th century, as a direct evolution from the vihuela, which also spawned the guitar's bastard cousin, the violin. As can be expected, the world of shreddage was still quite a long way off, as most early variations were pretty similar to today's Classical guitar. Multiple variations of this type of guitar were tossed about for a few centuries, finally refined in the 20th to the modern acoustic guitar (of course all of these were acoustic, but there isn't really another name for the standard Gibson/Yamaha/Ovation/etc. acoustic guitar).
Then, in the 1930s, the heavens opened up and the gods bestowed upon our pathetic little world the gifts of the amplifier and electric pickup, and metal was called forth from the depths to spread the good word of distortion and power chords and palm muting and...
Yes, that's right folks, the electric guitar was pretty lame itself when it first started out. Aside from the thoroughly awesome use it got in underground blues music, the public face of guitar was about as hardcore as a tickle fight with Tinkerbell. Commonly used as just a slimmer, louder acoustic, the electric guitars potential was hopelessly squashed by swing-era bands and soft-toned crooners. It wasn't until the 1960's that an accident with a dropped amplifier resulted in the discovery of distortion, and hooo boy the shit just hit the fan after that. The popularity of distorted guitar soon soared as rebellious kids were looking for the next "intense" or "rad" thing, and the Electric Guitar became, and still is, the face of modern Western music.
Guitars come in four primary forms, though there are obviously hundreds of different versions and spinoff of all of these.
This is the traditional form of guitar, used for soft, grandparent-friendly chamber music, funeral/wedding processions, and the surprisingly badass Spanish flamenco style, which, let me tell you, gets you all the ladies.
Making sweet, sweet music...for the ladies.
This type of guitar is typically slightly smaller, with nylon strings (meant to be finger-picked), a wide fretboard, and short neck. As seen above, the guitar is held on the left knee (for right-handed players) to allow for proper positioning of the fretting (left) hand.
The modern design was set by Antonio Torres Jurado in the 19th century, and they are now commonly hand-crafted in small workshops around the world, though most notably in Spain and Mexico (as made famous by the always-suave Antonio Banderas in Desperado and Once Upon a Time in Mexico.
This is the type of guitar most seen being played on the beach by random douchebags or used to back Christian praise music on every major avenue of every college campus, ever. Despite this, it is actually a great instrument, and can be fitted with a pickup and amplifier to mimic many of the effects of a full electric guitar. The acoustic has been used in nearly every genre of popular music, though now it is featured in Indie movie soundtracks above all else. It is most commonly seen in its 6-string form, though many artists use a 12-string version, the extra 6 exactly one octave above each string to provide a harmonic, chiming tone.
A twelve-string acoustic-electric being thoroughly ripped by John Butler.
The acoustic can be played sitting or standing, and is held on the right knee when played from a seated position, providing a more relaxed position for strumming, verses the elaborate picking of Classical. Acoustics are commonly larger than classical guitars, and feature steel strings on a narrow neck, with long fretboards and commonly-seen cutouts from the body (to allow the player to reach the highest frets easily).
Despite its less-than-promising start, electric guitars have become the staple of all rock and metal music today. The variety of sounds and effects that can be produced from this single instrument are rivaled only by the electric keyboard, but really after the 80's I think we've all had enough of that.
And you gave us Brett Michaels. Fuck you, 80's.
The power of the electric guitar stems from its pickups, which are a set of electromagnetic coils that turn the sound waves produced by the strings (which are naturally very quiet on solid-body guitars) into an electrical signal. This is transmitted to an amplifier, which essentially boosts and clarifies the sound, which then is output from the speaker, or "head". Though these are usually assumed to be the same thing, as most high school kids can't afford anything more than the 10-watt amp that came with their Target brand electric atrocity. However, professional-quality amplifiers are typically a separate unit from the massive stacks of speakers used in live concerts, perfectly designed for blasting sound at 1000 times the maximum level sustainable by human ears.
The physics of the electromagnetic pickup have allowed for all sorts of interesting techniques to be developed for the electric guitar. Some of the most notable and often-heard of these are:
Palm Muting - This is what produces the heavy, chugging sound found most commonly in Heavy Metal (this is a good example). The sound is achieved by lightly resting the palm of the picking hand against the strings near the bridge (basically the bottom end before they're attached to the body) and playing with quick, aggressive strokes.
Pinch Harmonics - A difficult but fuckin awesome technique that uses a suprising amount of harmonic physics to generate a high pitched "squeal". This is done by strumming a note, then gently placing the edge or the picking-hand thumb against the vibrating string. If the string is touched at a "node" in its harmonic oscillation (big words!) the pitch is immediately elevated to "holy shit" levels, which sounds absolutely sick when done properly.
Pick Slides - Kinda odd that all these begin with P, but whatever. Pick slides are often found at the beginning of songs or right before a sick solo or breakdown; the edge of the pick is slid along the length of the lower strings, producing a rasping, sliding high-to-low sound.
Synthesizer and Pedal Effects - The electrical signal of a guitar can be modified by a number of different synthesizers or pedals, which produce a wide range of effects such as the iconic "Wah" effect, the strange "Talk Box" (as heard in the beginning of Bon Jovi's "Living On a Prayer"...don't act like you don't know what I'm talking about), and the downright wrong "Synthesized backing choral voice" effect.