sometime around Bach, the triad became the common chord, and it is constructed by playing two notes a third apart and then adding another note a third above the higher note. A third is a musical interval, on a guitar, notes three frets apart ( ie open E and third fret on E ) or notes that are four frets apart ( ie open E and fourth fret on E ). The two notes that are three frets apart are called a minor third, and the notes four frets apart are called a major third.These are the "Official" classical names for the intervals. A major chord has a major third first ( ie E and G# ) followed by a minor third ( ie G# and B). I can do a complete tutorial on chord theory later. This only covers basic root position chords and doesn't cover inversions. If any of this makes sense, or if none of it makes sense, PM me and I will be able to explain it another way.

PS
It is harder to learn chord theory on guitar. It is easier on piano. The basic guitar chords you know, are not always voiced normally...

anyways, contact me if you still need help...