Originally Posted by: guitarpeachI've heard to it referred to as tonality but I'm not sure if thats right.[/quote]
Tonality is the overall correct term for Functional Harmony - or how musical objects (scales, chords) relate to each other. The more specific concept (and central "engine") within Tonality and Functional Harmony work is Voice Leading. Voice leading is regarding ALL the notes present in a piece of music as part of a melodic thread.

Originally Posted by: guitarpeach
I've done a little bit of work with the basic I IV V, although i never though of ii, iii, vi, and vii being minor until now. But what about chords like A7, dsus, and D/F# where do they come into play?

Same as any other note or chord: it's either structural or ornamental.

Structural means it has a specific function that helps relate one chord to the next. It can be a resolving 7th tone (the 7th in an A7 resolving to the major 3 in D - note G moving to F# - in A7 (V) - D (I) progression). It could be a common tone that is shared from one chord to the next in the progression (holding onto the F# as the melody note changing from a D to G major 7). Inversions, like D/F# (a first inversion D major) might be used as a functional way of getting from a G major to an E minor so that while the chords are "leaping around" viewed from their roots (G-D-E) the bass line makes them change smoothly by being linear (G-F#-E).

Ornamental means that you simply like the sound of it but it doesn't necessarily functional advance the melody or chord progression. This happens when you play a A major while changing it to Asus4-A-Asus2 and so forth.
[QUOTE=guitarpeach] Am I thinking about this to much, is it more just playing by ear or are the more steady rules to this?

There is no such thing as thinking too much. :) I did a view posts on this so what comes next is a copy paste summary.

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The Basics of Functional Harmony
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After learning and playing a good number of songs, music students will notice that certain chord progressions keep being used over and again. Many times I've heard the question, "Why do we use these three chords - I, IV, V - as the standard chord progression?" In order to understand the answer to this we need an overview of Functional Harmony.

Let's take a step back, start at the very beginning and check our premises.

Why do we bother to change chords at all in a song? Why not simply use one chord all the way through? It's possible to do, after all; and some songs actually do this. The reason we change chords and use more than one in a song is to provide variety and thereby make the music sounds as if it going somewhere. Hopeful somewhere purposeful. Once you truly understand that answer, it can help a great deal in making different chord progressions and thereby make your playing much more richly detailed and beautiful.

Functional Harmony is the science of understanding how changing chords can suggest goal-directed motion in music.

Building a chord on each note of the major scale results in the standard series of major and minor chords that are identified by Roman numerals:

I - "1 chord" is Tonic (or Root)
ii -"2 chord" is Sub-Dominant
iii - "3 chord" is Intermediate (or Mediant)
IV - "4 chord" is Sub-Dominant
V - "5 chord" is Dominant
vi - "6 chord" is Intermediate (or Sub-Mediant)
vii dim - "7 chord" is Dominant

Each of these chords has a specific function as follows:
Tonic - I
Intermediate - iii, VI (can also serve as substitute for I in deceptive cadence)
Sub-dominant - ii, IV
Dominant - V, vii diminished

Baroque, Classical, Romantic & Modern music - all follow this basic outline of Functional Harmony regarding chord progressions:

Tonic, then Intermediate, then Sub-dominant, then Dominant, return to Tonic.

This can, for example, be reduced to:

Tonic, Dominant, Tonic
Dominant, Tonic
Tonic, Intermediate, Dominant, Tonic
Tonic, Sub-dominant, Dominant, Tonic

In any of these cases, notice the primary goal is always the proper cadence:

Dominant, Tonic

It is also possible to prolong the progression, for example:

Tonic, Intermediate, Tonic, Sub-dominant, Intermediate, Dominant, Tonic.

Notice the goal remains the same: the Dominant to Tonic motion called a [/b]cadence[/b]. It is of course possible to not use the Dominant - Tonic resolution in a song (or part of a song). This is a useful effect in it's own right. It can make the music sound more open-ended, or ongoing. Or even restless or wandering, due to not having a proper resolution as a "resting point" or end goal.

From here we need to understand the concept of secondary dominant.

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The Basics of Secondary Dominant
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If you are in C major, you can construct a chord progression such as:

I, iii, ii, V, I
(being: C maj, E min, D min, G7, C maj)

In order to "fill out" the piece, make it more beautiful, richly complex & "prepare for the arrival" of the E min from the C maj you can do this:

I, ii of iii, V of iii, iii...& so forth
(being: C maj, F# min, B7, E min...)

So you have deviated from pure C major, and are temporarily in the key of E minor but only long enough to prepare for its arrival, then you return to key of C major. This is the concept called Secondary Dominant. It is a Dominant chord inserted into a sequence of chords that creates a temporary Dominant-Tonic cadential resolution goal that is not in the home key.

It is sometimes referred to as an Applied Dominant.

Notice the concept remains the same, though. The reason we use different chords is to provide variety & interest. And the reason we play certain chords in certain orders is to provide (or avoid) a sense of goal directed motion in the music.

This type of thing is not solely found in classical and jazz. It is the exact thing that happens in the beginning of the Beatles song "Yesterday":

G - F# min - B7 - E min
I - ii of vi - V of vi - vi

Typically, the secondary dominant is referred to in a chord progression as the "five of" whatever chord it is preparing. Because the vii dim chord can also be used as a dominant function you may also see "seven of" as a way of preparing for chord.

I really need to put this stuff in a lesson so I just link it all in the future. Hope this helps.