By adding extensions you can create nice chords, more complex-sounding - with extensions like the 9, 11 and 13. These extensions are usually played over dominant chords and are shaped like a triad.

In the triad you find the chosen extensions; over a C7(9 #11 13) you play a D, F#, A, which is in fact a Dmaj triad. Minor triads are used as well, as in C7(b9 #11 13), namely the F#mi triad. Theoretically we have 24 triads available, 12 major and 12 minor, but some will sound rather dissonant. The left hand complements with the basics: the seventh and the third.

Have fun practising,

Kino