A buffer system is a mixture of a weak acid and the salt of that acid's conjugate base (weak acids and bases can intermingle roles, and the counterpart to any weak acid or base is its conjugate acid or conjugate base). These systems resist changes in pH, making them very useful for biological systems that require their pH maintained within a specific range. The most effective buffering occurs when the pH of the solution is equal to the acid dissociation constant, or pKa.

An example of a buffer system in the body is blood, which contains carboxylic acid, a weak acid that returns the plasma to an equilibrium concentration whenever a new acid or base is introduced (up to a certain point, of course, it's not going to work very well if you inject hydrochloric acid straight into your veins, methinks).