What the are and why they are so common!
Chord: (Music) any group of 3 or more notes which may be named.
Now this might not apply to guitarists many of whom think TWO notes make a chord, as in power chord. (Note to guitarists - the rest of the music world disagrees with this definition. A "power chord" such as an A5 is held to be an "Interval" despite what you might have been told or read. This would make it really, really hard to comprehend music "theory.")
When I say "named" I mean has a common name. Some chords sound just plain lousy and thus don't get names.
Most all chords simplify to basic, three note chords. You can go through any chord list and x out anything you don't want to do this way and you'll still retain much of the "flavor" of the chord. The 9's, 11th's, and so on are mere condiments added to the basics. They do take a lot of time, time better spent learning some new material, I'd wager.
Also, despite some older books, the common keys are C, F, G, Bb, and D. I have found nothing to dissuade me of this. Older books use other, less common keys, and many a learned paper has been written why.
I place anything like this in the rack of "little rewards for a lot more work" and simply don't do it at all.
This is a good thing, because that means that there are a more reasonable number of common chords (a topic I cover thoroughly in my ebook about chords which I highly suggest you purchase at amazon if you are serious about chords! It's called How to Use Chords and Fakebooks by me, of course.) These are chords you CAN learn in a reasonable amount of time. For the first 5 keys I've listed there are 19 of them: 7 Major chords, 7 Minor chords, and 5 7th chords.
The key to playing these depends totally on what instrument you are playing. Of course, instruments that DO not make more than one or two notes can NOT play chords. The popularity of chord instruments like guitar, piano, and keyboard attests to strength of chords.
Chords are part of harmony, however, NOT melody and thus are not actually tunes (another name for a melody!) However, unless you want to hear a bunch of single note instruments you will have some sort of harmony.
That means chords. They are the accompaniment to almost everything written these days.
