A zwitterion (from the German, "hybrid ion") is an ion which can simultaneously act as an acid and a base.
Amino acids are common examples of zwitterions. Amino acids contain both acid (COOH) and base (NH2) groups. Amino-acids at physiological pH values predominate at an ionization state where the alpha-amino group is positively charged whereas the carboxyl group has a negative charge.
Zwitterions make good buffer solutions - they resist change to the pH of a solution by selective ionisation. In the presence of acids, zwitterions accept the hydrogen ions, removing them from the solution. With alkalis, zwitterions donate hydrogen ions in to the solution, again balancing the pH.