Learning Classical Guitar - What is a Chord

What's that chord? Chord is the key? ok, many of which already know, it is the key chord. But what exactly chord or key? I'm sure, not everyone knows.

Chord is the unity of the sound of three notes or more. not yet understand? C note played on strings 2, E on strings 4, and G in the string 5, one by one, well the result is a melodic flow. Now playing at the same time, that C major chord.

Chord on the guitar:

Actually there are so many chords on guitar (over 3000 if not mistaken). How can we memorize so many chords in my head? people say, memorize chord-chord aja we used to wear, the rest later. True, too, if you want to go into the pop genre, for what is familiar chord blues. But I say, know the chords? why? Just memorize the basic principles of chord.

There are basic principles that are used to form a chord, so that we can make our own version of the chord. How principle?