A zwitterion is a dipolar ion that is capable of carrying both a positive and negative charge simultaneously. The best example of this is amino acids. Amino acids have a basic NH2 group and an acidic COOH group. What usually happens is that the H from the COOH group (carboxylic acid end) migrates to the NH2 group; this causes what is known as a "dipole moment."
Selected zwitterion links: