'Add' in a chord name indicates that an additional note is added to the chord, like 'Cadd9' which means a C Major chord with an added 9th (D). 'Sus' stands for 'suspended', and 'sus2' or 'sus4' means the third of the chord is replaced by a 2nd or a 4th. 'Dim' stands for 'diminished', which indicates the chord is a diminished chord.