A buffer is composed of a weak acid molecule in equilibrium with its conjugate base and hydrogen ion (H+). An example of an acetic acid/acetate buffer is shown below.
CH3COOH < -------- > CH3COO- + H+
The acetic acid molecule (CH3COOH) acts as a store of base and acid ions.
The acetate ion (CH3COO-) acts like a base which accepts H+ and neutralizes them.
The hydrogen ion (H+) acts like an acid which accepts OH- and neutralizes them.