Music theory is the mechanics of music. We study music theory to identify the patterns and applications of sound, so that we can learn to manipulate them down the road.
Should I take the time to learn theory?
It's a great place to start. It helps when playing on or transposing between 2 instruments, and will help you develop intresting ways of applying chords and notes in your music endeavors.
There are musicians who have no formal training or music knolodge, mind you. And not all of them are Punk Musicians.
A scale is a combination of music notes. In most scales, there are a total of 7 different notes, followed by a note of the same kind but different pitch (As in a low A and a high A.). There are scales that have less or more notes than 7.
This is the A major scale: A, B, C#, D, E, F#, G#, A. In this scale, the root note (The note that determines what the scale is called.) is the A. Each note can be counted off based on its specific place in the scale. For example, B is the 2nd, the C# is the major 3rd, the E is the fifth. the G# is the major 7th, and so on.
Now we have the A minor scale: A, B, C, D, E, F, G, A. Notice how all the notes that had sharps (#) are now lacking them. This is becuase the minor scale flats (b, means to lower. As opposed to sharps, which raise.) the 3rd, 6th, and 7th of the major scale. In this scale, naming each note can still apply but the names of the notes will vary. In this scale, the C is the MINOR 3rd becuase of it's change to the minor scale. This applies the to the other notes as well, as the F is not a minor 6th and the G is now a minor 7. For more study on scales, click here.
Now let's start with chords. By definition, a music chords is a collection of 3 or more notes/pitches played together. You were probobly taught chords when you first started guitar, most likely major chords.
Major Chords: A major chord is a combination of the Root, third, and fifth notes of a scale. For example, the A major triad is composed of the notes A, C#, and E. The A is the root note, the C# is the major 3rd, and the E is the perfect 5th of the scale.
Minor Chords: Recall a minor scale has a flat 3rd, flat 6th, and flat 7th in it. And the major triad is 1-3-5. To make a minor chord, we flat the 3rd of the chord we're using. This makes the triad 1-b3-5. For example, the A minor triad would be A-C-E, as the C# has been made flat.
Suspended 2 Chords: In a Suspended 2 chord, abbreviated as "Sus2", we substitite the 3nd in the triad with a 2nd of a scale. This makes a 1-2-5 triad. In a Asus2 chord, this makes the notes A-B-E, as B is the 2nd note in both major and minor A scales.
Suspended 4 Chords: In the same way we made a Sus2 chord, we do the same to make a Sus4 chord. Only rather than substitute the 2nd note of a scale in, we use the 4th. This makes our triad 1-4-5. In a Asus4 chord, this makes the notes A-D-E.
7th Chords: A popular and common sight in blues and rock and roll, a 7th chord is usually a tetrad (4 notes.) but can sometimes be shortened to a triad or dyad (2 notes.). In these chords, we don't substitute the 3rd for anything. Rather, we add the 7th of a minor scale (A flat 7th compared to a major scale.) onto the triad/dyad of another chord. This can make a 1-3-5-b7 tetrad, or a 1-3-b7 triad. The fifth is not neccessarily needed to determine whether it is a major or minor chord. As an example, the A triad is A-C#-E. We add the minor 7th, a G, onto it and create A-C#-E-G. (You can still add onto a dyad of A-C# to make a traid of A-C#-G and still have a A7 chord.)
Major 7th Chords: Similiar in structure to the 7th, only we use the 7th of a major scale as opposed to the 7th of a minor chord. The same rules of applying this to a triad and dyad still apply here. In A, you can make a Tetrad of 1-3-5-7, and a Triad of 1-3-7. This makes the notes A-C#-E-G#, and A-C#-G# respectively.
Minor 7th Chords: Notice in the 7th chord that though we added a note from the minor scale (The flat 7th.) that it did not make it a minor chord. Also recall that to make a minor chord we substitute the 3 of a chord with a flat 3rd from a minor scale. The same applies in the maiking of a Minor 7th chord, whose tetrad is 1-b3-5-b7. In A, this makes the tetrad A-C-E-G.
Augmented Chords Augmented chords are intresting souding chords that have been used in many genres from Heavy Metal, to urban blues. It is created by adding in a sharp 5th (#5) to your chord. A Triad would be represented as "1-3-#5". In the key of A, this makes the notes "A-C#-F".
Diminished Chords Diminished chords are often "Sour" sounding chords used to end a musical phrase. It is created by adding a flat 3rd (b3) and a flat 5th (b5) to a root note. It's important to remember that a flat 5th (b5) is a tritone of a scale, a note which is often used in passing phrases and can add a ominous or blusey sound to your riffs. A Triad that represents a Diminished chord would be "1-b3-b5". In the key of A, this is "A-C-Eb".