Triads are groups of three notes derived from a key. In classical music, these notes are marked by their position in the scale. For example, a C major chord in the key of C would be marked by the symbol "I." A G major chord in the same key would be marked with "V."
Augmented and diminished triads often appear in minor keys. Augmented triads are often marked with a "C+" or "Caug." Diminished triads are marked with "C°," "Cdim," "Cm(♭5)," "Cmin(♭5)" or "C-(♭5)."
Seventh chords are chords that contain the seventh degree of the scale as well as the chord upon which it is based. Seventh chords can contain either a normal or lowered seventh. The lowered seventh is marked with "C7" or "Cm7," while the normal seventh chord is marked with "CM7."
Much like the seventh chord, the ninth chord contains an added ninth note. Major ninth chords are often marked with "CM9," "Cmaj9" or "C"9."
Eleventh chords are chords that contain an added eleventh note. Major and minor eleventh chords are often marked with "CmM11," "Cmmaj11," "CminM11," "Cminmaj11," "C'M11" or "C'maj11."