Guitar and Piano Theory
Well, at least it's short!
I've posted a few hubs where all the material is in the same key - if you check them out it might help you in understanding this post. In my opinion music theory as taught is always too complex. If you learn one key really well, all the relationships (chords, intervals, scales, inversions) can be transferred to all the other keys.
In other words, once you learn the pattern for one key, it can be transferred to all the other keys.
The numbers especially turn people off, but they are crucial - a number just tells you the distance from the starting note, so C6 just means a C chord with an A added-
C D E F G A
So C6 is C E G A (The C chord is C,E,G.Plus the sixth.)
C7 is C E G Bb
C maj7 is C E G B
For all these chords, a basic C chord (1,3,5) (C,E,G) has a note added to make a four-note chord. In practice, it's faster to count down from note 8, the same as note 1, but one octave higher.
The last couple of bullet points are guitar-centric, but the note sequence is the same for piano or keyboard, and also bagpipes!
Note names - there are no flats or sharps between B and C , E and F - all the other notes have them.
On piano, the seven different chords in any key have a fixed pattern:
Major, Minor, Minor, Major, Major, Minor, dim or min 7th flat 5
Shown in more detail in my hub Piano Chord Pictures, you don't need to read music!
In the key of C, just move the chord shape up one white note to the right, so C E G (C) becomes D F A, etc. (Dm) Repeat this move to get all the chords in C.
If you play guitar, it's almost essential to learn these theory basics on piano/keyboard.
Easy Music theory
The material shown here is all you need to start songwriting - and using Garageband for instance, you could be creating your own tunes in a very short time. If you take other songs apart and reassemble them it's a great way to learn fast - just as taking cars to bits will give you some insight into how they work.
So here's a plan for learning basic theory on keyboard or guitar:
Learn the harmonised scale in C, seven different chords that fit with the seven different notes in a major scale.
Then add sevenths to each chord. On piano this will use the same pattern of play one, miss one, four notes that move to the right one step at a time.
The final step is to try inversions, a C chord could have a C bass note, but also an E or a G in the bass, using notes from the chord. Very common in great songs by The Beatles and The Kinks (Waterloo Sunset)