A zwitterion is a
dipolar ion that is capable of carrying both a
positive and
negative charge simultaneously.
The best examples of this are 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 (ammonia) group; this causes what is known as a "dipole moment."
This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/