Triad
A triad is a three-note chord built from two thirds, one on top of the other. On the keyboard that is every other white key: C–E–G, F–A–C, G–B–D.
Building a triad is simple: take a note, skip a key, take the next, skip one more and take the third. The bottom note is the root, and it gives the chord its letter: C, F, G. On the staff a triad looks like a snowman: three notes on three lines in a row, or in three spaces in a row.
Triads, like scales, are major or minor. The difference is the middle note. A major triad has a major third (four half steps) at the bottom and a minor third (three) on top: C–E–G. A minor triad is the other way round, with the minor third at the bottom: A–C–E. In both, the outer notes make a fifth – seven half steps.
A triad can stand on any degree of the scale, but three matter most: on degree I the tonic (C in C major), on IV the subdominant (F), on V the dominant (G). Stack the same three notes in another order and you get an inversion.
What it sounds like
A major triad sounds bright and firm, a minor triad darker and sadder. The whole difference is one half step in the middle note.
Where it is taught: the studio’s interactive textbook, chapter 9, “Triads: C, F, G”.