An ion is an atom or molecule with a positive or negative electrical charge. A zwitterion is a molecule with both positive and negative charges, but with a net charge of zero. Atoms are composed of positively charged subatomic particles called protons, negatively charged electrons, and neutral particles called neutrons. When the number of protons is not equal to the number of neutrons, an atom will have a positive or negative charge. Negatively charged atoms or molecules are called anions, whereas positively charged atoms and molecules are known as cations.

Acids and bases may exist as ions, or as uncharged molecules. At neutral pH levels, both acids and bases exist as ions. Most zwitterions are ampholytes - molecules that contain both acidic and basic groups. Amino acids are a classic example; they have both acidic and basic groups in a single molecule. Because both the acid and the base are charged at neutral pH, the net charge of the amino acid will be zero.

Because of its electrical charges, a zwitterion will be highly soluble in water, and quite insoluble in organic solvents. Water molecules are polar because of the electrical differences between the hydrogen and oxygen molecules in water; that is, the oxygen atoms in water are attracted to cations and the hydrogen atoms in water are attracted to anions. When a zwitterion is dissolved in water, the hydrogen atoms in water immediately surround the negatively charged group of the zwitterion and the oxygen atoms in water immediately surround the positively charged group of the zwitterion, thereby thoroughly dissolving it.

In addition to amino acids, some other common types of zwitterions are buffers, detergents, and drugs. The fact that these types of molecules are zwitterions, and therefore soluble in water is what makes them so useful. When a zwitterion is stored in solid, dry form, for example in a pill, it will be in the form of a neutral molecule or a salt. When the zwitterion is eaten or placed in water, it takes on its ionic, or charged form, and becomes dissolved in water.

While it is true that most amino acids are neutral in charge, it is important to note that some amino acids have a third charged group, giving them an overall positive or negative charge. For this reason, proteins, which are composed of amino acids, generally have a net negative, or net positive charge.

The term zwitterion comes from the German word "zwitter." It means hybrid. The prefix zwi- means double, similar to the Old English prefix twi-.