Hmmm . . . we've had these discussions on here before. If you want a comprehensive explanation, check some old threads or the tricks section.

I'll give you a quick run-down -

5th - commonly called a powerchord. Just the root and the 5th.

7th - there are basically two types of sevenths (dominants and majors). A dominant seventh has the seventh tone from the minor scale, and a major seventh has the seventh tone from the major scale.

9th - Take a dominant 7th and add the 2nd.

Augmented - From a major triad, raise the 5th a half step.

Diminished - From a major triad, lower the 5th a half step.