Hey people!

I'm sure this has been asked a time or two, but I didn't quite find anything. What I'd like to know is some theory background to chord variations and more "odd" chords used in songs. I'm talking about for example 7-chords, SUS Chords, Maj7 chords, and also odd chords irrelevant to the key such as the chord E in C Major (which is IIIMajor).

What I'm after here is some theory background on this that can help in better understanding when and why these are used. Obviously they sound good at the spot where they are used, but that's a little thin for me. I want to know more. I think I remember having heard for example that Isus4 chords adds a suspension that "points toward" IV. Stuff like that. Hope you get what the heck I mean lol

Tagging this post as i'm also interested and would like to know more.
dude, chiggity-check out Neapolitin chords, augemented 6th chords, and the chromatic V/V or IV/V chords (my favorites).
The chords used depend on what you want in the progression. V instead of v is often used in minor key cause it wants to pull back strongly to i (the major third in V is a semitone lower than the tonic). You can subsitute chords, for example the tritone sub where the 3rd and 7th switch places with a different root or a diminished 7 sub instead of V. With both you can create chromatic root progressions which can sound awesome. To give an example of those:

normal C major progression: ii-V-I = Dm-G7-Cmaj7
tritone sub instead of V: ii-subV-I = Dm-Db7-Cmaj7
diminished 7 sub instead of V: ii-subV-I = Dm-Bdim7-Cmaj7

Also borrowed chords from the parallel minor/major happens a lot. iv instead of IV, III instead of iii, what you mentioned earlier.

Then you have the extensions you can use to spice the chords up, most of them are used to creat as much tension as possible so that the resolution becomes stronger.
Well it may be a bit difficult to understand cause no one here can show you what they mean. A seventh chord is nothing more than adding the seventh degree to the chord. In C Major the I chord would be ...C, E, G. Well if you add the seventh degree it would be ... C, E, G, B. This works for all chords, be it the I, ii, iii, IV, V, vi, or vii diminished. a iii7 chord in C Major would be ...E, G, B, D.