By definition a buffer is a substance (usually a weak acid and its conjugate base or a weak acid and its conjugate base)) which is added to a sample to avoid changes in its ph. A buffer must contain the chemical species for “neutralizing” added amounts of acid or base. For example, if a buffer was a solution of a weak acid and its conjugate base it would contain acetic acid and sodium acetate or a weak base and conjugate acid it would contain ammonia and ammonium chloride. If there is a lot of acid or alkali added then the buffer will not be able to cope with such a change and will no longer able to maintain the pH.
Buffers are most effective in the range PH = pK’a ± 1. Outside the range the concentration of either the acid or the conjugate base is too small to effectively resist the effect of added hydrogen or hydroxide ions.