Chords that include 9ths, 11ths, 13ths are usually referred to as extended chords. Extended chords are generally used to enhance smaller chords, such as triads and 7th chords. This kind of thinking can make a chord progression more interesting and sound more musically sophisticated it simply gives you more to listen to.
One very important featue of jazz harmony is that chords can be freely substituted for other chords within their "family". We can think of there being three basic "families" major, minor, and dominant. In other words, if the written chord progression contains a CMaj7 chord, you may replace it with a C6, CMaj9, CMaj13, or any other chord in the CMajor family. A Cmin7could be replaced with a Cmin6, Cmin11, Cmin13 or any other Cminor chord. With dominant chords the same idea holds true. A C7 could be replace with C9, C11, C13. Many students are suprised that this is possible. Yes it will change the sound of the tune and yes, that is perfectly alright. The chords you choose when playing a song help to define your own individual sound and style. After some these voicings become easy for you, try using them in songs you already know. Experiment freely, but let good taste dictate your choices.
=============================================
Major 9th Chords
The formula of the major 9th chord (Maj9) is Root-3-5-7-9. In the key of C, that's C-E-G-B-D. In F it's F-A-C-E-G.
===================================================
Minor 9th Chords
The formula for a minor 9th chord (min9) is Root b3-5-b7-9. In the key of G it's G-Bb-D-F-A. In D it's D-F-A-C-E.
========================================================
Dominant 9th Chords
The formula for a dominant 9th chord (9) is Root-3-5-b7-9. In the key of Eb that's Eb-G-Bb-Db-F. In Bb it's Bb-D-F-Ab-C.
