Maybe I could answer that one. I have studied the physics of sound and can concur with flatliner.
A vibrating string, for example the low E string, will sound its fundamental frequency, the E, by vibrating along its whole length. (Say 25 inches for the sake of example). However, the string also vibrates in other "modes" such that it appears to vibrate along half its length, third its length, quarter its length, fifth its length and so on. These are called higher harmonics or overtones*. Each higher "harmonic" (or overtone) is weaker than the previous. So you tend to hear the lowest (fundamental) tone and the presence or absence of the higher harmonics colours the sound you hear. For example, playing near the bridge encourages higher harmonics and consequently colours the tone of the note.
Now the tricky bit. The harmonics create notes which relate to the fundamental (lowest tone) in a mathematical sequence. The 2nd harmonic (where the string vibrates along half its length) is twice the frequency of the fundamental. This creates a note an octave higher than the fundamental. This note sounds with the fundamental and colours the tone. The next harmonic (string vibrates in 3rds) produces a note 3 times the frequency of the fundamental, and is an octave plus a fifth (13 notes) above the fundamental. So if the fundamental was a C, the 2nd harmonic is C above that and the 3rd is the G above that. It goes on. The next harmonic produces the octave above that, and the 5th produces the third of the chord (the E) above that.
Guitarists can force these harmonics to sound by touching a finger on the vibrating string.
If you touch at the 12th fret you force the string to vibrate in two halves, producing the 2nd harmonic, an octave above the fundamental. If you touch the string at the 4th fret you force the 5th harmonic, which is the 3rd of the chord. (On the bottom E string you would get a G#)
If your guitar string is is 25 inches long, the 5th harmonic (the 3rd of the chord) is encouraged by playing or strumming about 7 or 8 inches from the bridge. On my acoustics, this is just above the sound hole. Plucking the E string hard here, and listening, one can hear the 3rd of the chord (the G#) sounding quite clearly.
The same applies to electrics.
Hope this helps.
* There is a lot of confusion and disagreement over the meaning of the words overtone and harmonic. I have used the convention where the lowest note (fundamental) is also called the 1st harmonic. The 2nd harmonic, the octave above, is referred to as the 1st overtone, and so on.