Guitar done

Guitars. You see them everywhere.  Even I have a mini-Martin acoustic (that doesn't get played as much as it should...)

Because you see them so often, maybe you don't realize what's going on when someone plays one.  Acoustic guitars are the guitars you see at campfires or college campuses (or anywhere electricity is not, really).  They are hollow, have six strings, a fretted fingerboard, and some type of sound hole.  The hole can be just a round hole (most common) between the fingerboard and the bridge (where the ends of the strings are attached) or several smaller holes arranged around the fingerboard.  They can even be shaped like the f-holes like you see on violins and cellos.  The hollow body of the guitar acts as a resonator and helps to make the sounds of the vibrating strings (because sound is made by vibrations!) louder.

Electric guitars use electricity (of course) to power the pickups (that usually consist of some kind of magnet wrapped in copper wires) which, in turn, transform the signal into another kind of signal that can be amplified or recorded.  Basically, the pickups take the sound vibrations and turn them into an alternating current that can be sent through a cable.  Once the current is past the pickups it can be sent through amplifiers, distortion pedals, tuners, recorders, and in some cases, a preamplifier, that strengthens a low-level signal.

There are also guitars known as acoustic-electric guitars.  These guitars are made like acoustic guitars but have pickups and electronics installed to allow the player to send the sound through pedals or amps.  Sort of a hybrid. 

The standard tuning of guitars, from low to high, is E A D G B E.  I remember this with this mnemonic: Every Ant Did Good But Ernie.  Some guitars players retune their guitars to help with chord fingerings or to change the sound.