How does neutralization reaction differ from using a buffer?

Neutralization usually refers to any reaction of acid and base to achieve a pH at or near 7, but a buffer solution always consists of a solution containing significant concentrations of both a weak acid and its conjugate base for the purpose of maintaining a constant pH value (not necessarily near 7).

A buffer solution maintains a nearly constant pH (a measure of hydronium ion concentration or acidity). It is able to withstand the addition of small amounts of strong base by reaction that converts a small amount of the buffer weak acid to conjugate base. Or, it can withstand addition of small amounts of strong acid by converting some of the buffer conjugate base to its acid form. If the ratio of concentrations of the buffer weak acid and conjugate base is not altered much, then the pH remains nearly constant.

Note that a buffer solution can be prepared from a solution of weak acid by adding enough strong base to neutralize about half of the weak acid to its conjugate base form (for example, converting half the concentration of acetic acid to acetate anion). Alternatively, a beffer can be formed by adding strong acid to a solution of weak base to neutralize about half of the concentration to its conjugate acid form (e.g., converting half of the concentration of ammonia to ammonium cations).