acetic acid = CH3COOH(aq) , acetate = CH3COO-(aq) and proton = H+(aq)
so...
CH3COOH(aq) <-> CH3COO-(aq) + H+(aq)
this is the dissociation of acid, therefore, it will yield H+ ions in solution, which are ACIDIC
then... for the acetate buffer, recall that
CH3COO-(aq) is a weak conjugate base, so, it can accept H+ if in excess
so
H+(aq) + CH3COO-(aq) <--> CH3COOH(aq)
therefore, addition of excess H+, will form CH3COOH slowly.. i.e. th epH will not increase drstically
Addition of OH- ins, from a strong base:
OH-(aq) + H+(aq) <--> H2O(l)
then, OH- is apdily converted to H2O, which is not basic, so pH won't change drastically.