Piano chords are really no different than any other chords played on a different instrument. Piano chords are based off of the foundation of music scales and thus you must have a basic understanding of scales and intervals first. If you do not understand the basics of intervals, such as half tones and whole tones, than you must learn about them before trying to progress on to piano chords.
To begin with the basics of piano chords, we must start with the most fundamental chord of them all the triad. A triad is basically exactly what it sounds like; it is build upon three notes in a chord. To be honest, not matter what notes you play as long as you play 3 notes together, you will be playing some kind of chord. However this piano lesson is here to teach you about musical sounding chords.
First we must explain a bit of a numbering system before starting. When you play the diatonic scale of C major, which would be all the white keys, you can count the notes in sequential order. C would be 1, D would be 2, E would be 3 and so on.
To construct a chord all you basically do is play three notes as stated before, however the basic rule is that you play a white key, skip the next white key, play the next white key, skip the next white key and play the next white key. So if you play a C, you would skip the D and play the E, skip the F and play the G.
However this is where understanding intervals come into play. If you played the chord CEG, the interval from C to E would be two whole steps. However the interval from E to G would be one whole step and one half steps. This interval formula while playing a piano chord is known as a major chord. When the first note of a chord to the third note of a chord is two whole steps and the third note to the fifth note is one whole step and one half steps the chord is major.
So what is a minor chord? Just the opposite! If the first note to the third note is one and a half steps and the third to the fifth is two whole steps, it will be a minor chord. An example of this would be an EGB triad. E to G is one and a half steps but G to B is two whole steps. So if you played a triad on all the white keys, by skipping every other white key, starting on E the chord would be minor.
A fun exercise to do is to play basic triads on only white keys, by holding down three white keys, remember always skip a key on the keyboard. While playing the chords, take note of the notes that are being played and listen to the sound. Also take note of the intervals and try to guess which type of chord they are.