In Jazz, Blues, Funk, Rock, Latin and many other styles there are chords used which contain more than three notes. By taking a triad and adding other notes on top of it, a variety of other chords can be created. The most common of these are seventh chords.
A seventh chord can be created by adding the seventh note of a mode to the basic triad. The type of seventh chord this creates will depend on the notes contained in both the mode it is derived from and the basic triad. This can be confusing at first, so it is worth looking at the theory of chords and modes to identify the basic principles of chord and mode construction.