#1
So what is the difference between an 9 chord say, and an add 9?

And do all extended chords have a seventh in them?
#2
Writing C9 means you use all the chord note up to that 9th- 1,3,5,b7 and the 9.
Add9 - 1,3,5,9

So it's a matter of build or add....or soemthing...
Schecter C1 Classic
Fender Vintage -57 Ri Stratocaster
Fender Blues JR w/ 12"Cannabis Rex
Mad Professor Sky Blue OD
Wampler Ego Compressor
TC Electronics Stereo Chorus/Flanger
#3
Alot of extended chords have 7ths in and they often help define the chord for example a maj7 will have a 1 3 5 7 wheres a min7 will have a 1 b3 5 b7 and a dominant 7 will have a 1 3 5 b7. This can be extended to 9ths and other extended chords. In your example a 9th chord, i will assume you mean maj9, would be 1 3 5 7 9 (i think) whereas a add9 would be 1 3 5 9, the 9 is added without the use of any of the previous extensions eg 7. I think in further extensions like 11s and 13s it is not so important to have the add as it not necessarily possible to play all the notes so you can miss out certain notes and have the same name still. Sory for being confusing, my writing is terrible!