Cadd9 is a C major chord with an added 9th note. It is played with the notes C, E, G, and D.
Csús2 is a C suspended 2nd chord. It is played with the notes C, E, and G. The 2nd note (D) is suspended (not played).
The two chords have different sounds and functions in music. Cadd9 is a more open and brighter sounding chord, while Csús2 has a more tense and suspended sound. Cadd9 is often used as a jazz chord, while Csús2 is often used in blues and rock music.