I have a question relating to the naming of chords:

Why is a C chord a C chord and not a G chord since there are 2 G's and 2 C's and in fact a 3rd G could be added on the 6th string as well? This question would I think apply to the naming of any chord, but not sure.
It's to do with the note to which the chord resolves. Like, a G chord is I in the key of G, but a C chord will always be IV and resolve to C no matter how many G's you cram in there.

Edit: I messed up; read again
Well a C major chord has CEG as the triad, and a G chord has GBD as the triad. So while a c chord could have a bunch of G's, a G chord couldn't have any C's (without it being an 11th chord anyway.)
Sometimes its all how its played. For example, a C6 has the same notes as an Amin7! So it's important that you have at least a bass player playing a C so you know what chord it is. Of course sometimes it doesn't matter what its called.

Chords are defined as a stack of third intervals based around the root of the chord. If you take the C major scale and use the C as your root then a third up from there would be E(third note of the scale) then if you go up another third you would get G(fifth note of the scale) so from that you know that a C major chord will contain the notes C-E-G.

The order of the notes or however many times they occur in a particular chord doesn't matter, so you can arrange C, E, & G however you want to & it will still be a C major chord. Like cjgallagher1 said, C major contains C-E-G while G major contains G-B-D which comes from building the chord from the G major scale.

Of course once you move past the simple triads into seventh chords & beyond it gets more complicated & you will usually have to look at the context of the chord to correctly name it since various chords will contain the same notes. Pulling from cjgallagher1's post again the difference between a C6 & an Amin7 would depend on the chord's function in a progression.
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Thanks guys, this clears up a lot!